A Population-Based Matched-Sibling Analysis Estimating the Associations Between First Interpregnancy Interval and Birth Outcomes

Effects of short inter-pregnancy/birth interval on adverse perinatal outcomes in Asia-Pacific region: A systematic review and meta-analysis

BACKGROUND

Short inter-pregnancy or birth interval is associated with an increased risk of adverse perinatal outcomes. However, some emerging evidence questions this association and there are also inconsistencies among the existing findings. This study aimed to systematically review the evidence regarding the effect of short inter-pregnancy or birth intervals on adverse perinatal outcomes in the Asia-Pacific region.

METHODS

A comprehensive search of five databases was conducted targeting studies published between 2000 to 2023. Studies that reported on short inter-pregnancy or birth interval and examined adverse perinatal outcomes, such as low birthweight (LBW) preterm birth (PTB), small for gestational age (SGA), and neonatal mortality were included and appraised for methodological quality using the Joanna Briggs Institute critical appraisal tools. Three reviewers independently screened the studies and performed data extraction. Narrative synthesis and meta-analyses were conducted to summarise the key findings.

RESULTS

A total of 41 studies that fulfilled the inclusion criteria were included. A short-interpregnancy interval was associated with an increased risk of low birthweight (odds ratio [OR] = 1.65; 95%CI:1.39, 1.95), preterm birth (OR = 1.50; 95%CI: 1.35, 1.66), and small for gestational age (OR = 1.24; 95%CI:1.09, 1.41). We also found elevated odds of early neonatal mortality (OR = 1.91; 95%CI: 1.11, 3.29) and neonatal mortality (OR = 1.78; 95%CI: 1.25, 2.55) among women with short birth intervals.

CONCLUSION

This review indicates that both short inter-pregnancy and birth interval increased the risk of adverse perinatal outcomes. This underscores the importance of advocating for and implementing strategies to promote optimal pregnancy and birth spacing to reduce the occurrence of adverse perinatal outcomes. Reproductive health policies and programs need to be further strengthened and promote access to comprehensive family planning services and increase awareness about the importance of optimal pregnancy and birth spacing.

Interpregnancy Interval After Healthy Live Birth and Subsequent Spontaneous Abortion

Importance

Many studies have reported that the interpregnancy interval (IPI) is a potential modifiable risk factor for adverse perinatal outcomes. However, the association between IPI after live birth and subsequent spontaneous abortion (SA) is unclear.

Objective

To investigate the association of IPI after a healthy live birth and subsequent SA.

Design, Setting, and Participants

This prospective cohort study used data from 180 921 women aged 20 to 49 years who had a single healthy live birth and planned for another pregnancy and who participated in the Chinese National Free Prepregnancy Checkups Project from January 1, 2010, to December 31, 2020. Statistical analysis was conducted from June 20 to October 5, 2023.

Exposure

Interpregnancy interval, defined as the interval between the delivery date and conception of the subsequent pregnancy, was categorized as follows: less than 18 months, 18 to 23 months, 24 to 35 months, 36 to 59 months, and 60 months or longer.

Main Outcomes and Measures

The main outcome was SA. Multivariable-adjusted odds ratios (ORs) were calculated by logistic regression models to examine the association between IPI and the risk of SA. Dose-response associations were evaluated by restricted cubic splines.

Results

The analyses included 180 921 multiparous women (mean [SD] age at current pregnancy, 26.3 [2.8] years); 4380 SA events (2.4% of all participants) were recorded. A J-shaped association between IPI levels and SA was identified. In the fully adjusted model, compared with IPIs of 18 to 23 months, both short (<18 months) and long (≥36 months) IPIs showed an increased risk of SA (IPIs of <18 months: OR, 1.15 [95% CI, 1.04-1.27]; IPIs of 36-59 months: OR, 1.28 [95% CI, 1.15-1.43]; IPIs of ≥60 months: OR, 2.13 [95% CI, 1.78-2.56]). Results of the subgroup analysis by mode of previous delivery were consistent with the main analysis.

Conclusions and Relevance

This cohort study of multiparous women suggests that an IPI of shorter than 18 months or an IPI of 36 months or longer after a healthy live birth was associated with an increased risk of subsequent SA. The findings are valuable to make a rational prepregnancy plan and may facilitate the prevention of SA and improvement in neonatal outcomes.

Parental Perceptions of Counseling Regarding Interpregnancy Interval after Stillbirth or Neonatal Death

OBJECTIVE

Although guidelines exist regarding optimal interpregnancy interval (IPI) after live birth, both optimal IPI and counseling regarding recommended IPI (rIPI) after stillbirth or neonatal death is not well established. Our goal was to describe the counseling bereaved parents receive regarding IPI, parents' reactions to that counseling, and actual IPI after loss.

STUDY DESIGN

Bereaved parents who had a previous pregnancy result in stillbirth or neonatal death participated in a web-based survey. Questions included demographics, details of stillbirth or neonatal death, IPI counseling, and pregnancy after loss. Demographic information, rIPI, and ac'tual IPI were reported using descriptive statistics. The Wilcoxon's rank sum test was used to test the association between rIPI and mode of delivery. The Spearman's correlation was used to test the association between rIPI and maternal age.

RESULTS

A total of 275 surveys were analyzed. Mean gestational age of stillbirth delivery was 33.1 (standard deviation: 6.6) weeks. A total of 29% delivered via cesarean. Median rIPI was 6 (interquartile ratio [IQR]: 2-9) months, with the primary reason for IPI reported as the need to heal (74%). Delivery via cesarean was associated with longer rIPI, 9 versus 4.2 months (p < 0.0001). Maternal age was not associated with rIPI. Of 144 people who pursued pregnancy again, median time until attempting conception was 3.5 (IQR: 2-6) months. Median actual IPI was 6 (IQR: 4-10) months.

CONCLUSION

Bereaved parents receive a wide range of counseling regarding rIPI. The majority receive rIPI and pursue actual IPI shorter than current national and international recommendations for optimal IPI.

KEY POINTS

· There is variation in IPI recommendation after stillbirth/neonatal death.. · Cesarean birth is associated with longer IPI recommendation, but maternal age is not.. · Median IPI after stillbirth or neonatal death was short: 6 (IQR: 4-10) months..

Short Interpregnancy Intervals Among Women Experiencing Homelessness in Colorado

Importance

Short interpregnancy intervals (SIPIs) are associated with increased risk of adverse maternal and neonatal outcomes. Disparities exist across socioeconomic status, but there is little information on SIPIs among women experiencing homelessness.

Objective

To investigate (1) differences in rates and characteristics of SIPIs between women experiencing homelessness and domiciled women, (2) whether the association of homelessness with SIPIs differs across races and ethnicities, and (3) whether the association between SIPIs of less than 6 months (very short interpregnancy interval [VSIPIs]) and maternal and neonatal outcomes differs between participant groups.

Design, Setting, and Participants

This cohort study used a Colorado statewide database linking the Colorado All Payer Claims Database, Homeless Management Information System, death records, and infant birth records. Participants included all women who gave birth between January 1, 2016, and December 31, 2021. Data were analyzed from September 1, 2022, to May 10, 2023.

Exposures

Homelessness and race and ethnicity.

Main Outcomes and Measures

The primary outcome consisted of SIPI, a binary variable indicating whether the interval between delivery and conception of the subsequent pregnancy was shorter than 18 months. The association of VSIPI with maternal and neonatal outcomes was also tested.

Results

A total of 77 494 women (mean [SD] age, 30.7 [5.3] years) were included in the analyses, of whom 636 (0.8%) were women experiencing homelessness. The mean (SD) age was 29.5 (5.4) years for women experiencing homelessness and 30.7 (5.3) years for domiciled women. In terms of race and ethnicity, 39.3% were Hispanic, 7.3% were non-Hispanic Black, and 48.4% were non-Hispanic White. Associations between homelessness and higher odds of SIPI (adjusted odds ratio [AOR], 1.23 [95% CI, 1.04-1.46]) were found. Smaller associations between homelessness and SIPI were found among non-Hispanic Black (AOR, 0.59 [95% CI, 0.37-0.96]) and non-Hispanic White (AOR, 0.57 [95% CI, 0.39-0.84]) women compared with Hispanic women. A greater association of VSIPI with emergency department visits and low birth weight was found among women experiencing homelessness compared with domiciled women, although no significant differences were detected.

Conclusions and Relevance

In this cohort study of women who gave birth from 2016 to 2021, an association between homelessness and higher odds of SIPIs was found. These findings highlight the importance of conception management among women experiencing homelessness. Racial and ethnic disparities should be considered when designing interventions.

Birth spacing and risk of adverse pregnancy and birth outcomes: A systematic review and dose-response meta-analysis

INTRODUCTION

The association between extreme birth spacing and adverse outcomes is controversial, and available evidence is fragmented into different classifications of birth spacing.

MATERIAL AND METHODS

We conducted a systematic review of observational studies to evaluate the association between birth spacing (i.e., interpregnancy interval and interoutcome interval) and adverse outcomes (i.e., pregnancy complications, adverse birth outcomes). Pooled odds ratios (ORs) with 95% confidence intervals (CI) were calculated using a random-effects model, and the dose-response relationships were evaluated using generalized least squares trend estimation.

RESULTS

A total of 129 studies involving 46 874 843 pregnancies were included. In the general population, compared with an interpregnancy interval of 18-23 months, extreme intervals (<6 months and ≥ 60 months) were associated with an increased risk of adverse outcomes, including preterm birth, small for gestational age, low birthweight, fetal death, birth defects, early neonatal death, and premature rupture of fetal membranes (pooled OR range: 1.08-1.56; p < 0.05). The dose-response analyses further confirmed these J-shaped relationships (pnon-linear  < 0.001-0.009). Long interpregnancy interval was only associated with an increased risk of preeclampsia and gestational diabetes (pnon-linear  < 0.005 and pnon-linear  < 0.001, respectively). Similar associations were observed between interoutcome interval and risk of low birthweight and preterm birth (pnon-linear  < 0.001). Moreover, interoutcome interval of ≥60 months was associated with an increased risk of cesarean delivery (pooled OR 1.72, 95% CI 1.04-2.83). For pregnancies following preterm births, an interpregnancy interval of 9 months was not associated with an increased risk of preterm birth, according to dose-response analyses (pnon-linear  = 0.008). Based on limited evidence, we did not observe significant associations between interpregnancy interval or interoutcome interval after pregnancy losses and risk of small for gestational age, fetal death, miscarriage, or preeclampsia (pooled OR range: 0.76-1.21; p > 0.05).

CONCLUSIONS

Extreme birth spacing has extensive adverse effects on maternal and infant health. In the general population, interpregnancy interval of 18-23 months may be associated with potential benefits for both mothers and infants. For women with previous preterm birth, the optimal birth spacing may be 9 months.

The influence of birth outcomes and pregnancy complications on interpregnancy interval: a quantile regression analysis

PURPOSE

To ascertain whether adverse pregnancy outcomes at first pregnancy influence subsequent interpregnancy intervals (IPIs) and whether the size of this effect varies with IPI distribution METHODS: We included 251,892 mothers who gave birth to their first two singletons in Western Australia, from 1980 to 2015. Using quantile regression, we investigated whether gestational diabetes, hypertension, or preeclampsia in the first pregnancy influenced IPI to subsequent pregnancy and whether effects were consistent across the IPI distribution. We considered intervals at the 25th centile of the distribution as 'short' and the 75th centile as 'long'.

RESULTS

The average IPI was 26.6 mo. It was 0.56 mo (95% CI: 0.25-0.88 mo) and 1.12 mo (95% CI: 0.56 - 1.68 mo) longer after preeclampsia, and gestational hypertension respectively. There was insufficient evidence to suggest that the association between previous pregnancy complications and IPI differed by the extent of the interval. However, associations with marital status, race/ethnicity and stillbirth contributed to either shortening or prolonging IPIs differently across the distribution of IPI.

CONCLUSION

Mothers with preeclampsia and gestational hypertension had slightly longer subsequent IPIs than mothers whose pregnancies were not complicated by these conditions. However, the extent of the delay was small (<2 mo).

Weighting of risk factors for low birth weight: a linked routine data cohort study in Wales, UK

OBJECTIVE

Globally, 20 million children are born with a birth weight below 2500 g every year, which is considered as a low birthweight (LBW) baby. This study investigates the contribution of modifiable risk factors in a nationally representative Welsh e-cohort of children and their mothers to inform opportunities to reduce LBW prevalence.

DESIGN

A longitudinal cohort study based on anonymously linked, routinely collected multiple administrative data sets.

PARTICIPANTS

The cohort, (N=693 377) comprising of children born between 1 January 1998 and 31 December 2018 in Wales, was selected from the National Community Child Health Database.

OUTCOME MEASURES

The risk factors associated with a binary LBW (outcome) variable were investigated with multivariable logistic regression (MLR) and decision tree (DT) models.

RESULTS

The MLR model showed that non-singleton children had the highest risk of LBW (adjusted OR 21.74 (95% CI 21.09 to 22.40)), followed by pregnancy interval less than 1 year (2.92 (95% CI 2.70 to 3.15)), maternal physical and mental health conditions including diabetes (2.03 (1.81 to 2.28)), anaemia (1.26 (95% CI 1.16 to 1.36)), depression (1.58 (95% CI 1.43 to 1.75)), serious mental illness (1.46 (95% CI 1.04 to 2.05)), anxiety (1.22 (95% CI 1.08 to 1.38)) and use of antidepressant medication during pregnancy (1.92 (95% CI 1.20 to 3.07)). Additional maternal risk factors include smoking (1.80 (95% CI 1.76 to 1.84)), alcohol-related hospital admission (1.60 (95% CI 1.30 to 1.97)), substance misuse (1.35 (95% CI 1.29 to 1.41)) and evidence of domestic abuse (1.98 (95% CI 1.39 to 2.81)). Living in less deprived area has lower risk of LBW (0.70 (95% CI 0.67 to 0.72)). The most important risk factors from the DT models include maternal factors such as smoking, maternal weight, substance misuse record, maternal age along with deprivation-Welsh Index of Multiple Deprivation score, pregnancy interval and birth order of the child.

CONCLUSION

Resources to reduce the prevalence of LBW should focus on improving maternal health, reducing preterm births, increasing awareness of what is a sufficient pregnancy interval, and to provide adequate support for mothers' mental health and well-being.

Short interpregnancy interval can lead to adverse pregnancy outcomes: A meta-analysis

BACKGROUND

The evidence of some previous papers was insufficient in studying the causal association between interpregnancy interval (IPI) and adverse pregnancy outcomes. In addition, more literature have been updated worldwide during the last 10 years.

METHODS

English and Chinese articles published from January 1980 to August 2021 in the databases of PubMed, Cochrane Library, Ovid, Embase, China Biology Medicine disc (CBM), and China National Knowledge Infrastructure (CNKI) were searched. Then following the inclusion and exclusion criteria, we screened the articles. Utilizing the Newcastle-Ottawa Scale (NOS), we evaluated the quality of the included articles. The literature information extraction table was set up in Excel, and the meta-analysis was performed with Stata 16.0 software (Texas, USA).

RESULTS

A total of 41 articles were included in the meta-analysis, and NOS scores were four to eight. The short IPI after delivery was the risk factor of preterm birth (pooled odds ratio 1.49, 95% confidence interval 1.42-1.57), very preterm birth (pooled OR: 1.82, 95% CI: 1.55-2.14), low birth weight (pooled OR: 1.33, 95% CI: 1.24-1.43), and small for gestational age (pooled OR: 1.14, 95% CI: 1.07-1.21), offspring death (pooled OR: 1.60, 95% CI: 1.51-1.69), NICU (pooled OR: 1.26, 95% CI: 1.01-1.57), and congenital abnormality (pooled OR: 1.10, 95% CI: 1.05-1.16), while was not the risk factor of gestational hypertension (pooled OR: 0.95, 95% CI: 0.93-0.98) or gestational diabetes (pooled OR: 1.06, 95% CI: 0.93-1.20).

CONCLUSION

Short IPI (IPI < 6 months) can lead to adverse perinatal outcomes, while it is not a risk factor for gestational diabetes and gestational hypertension. Therefore, more high-quality studies covering more comprehensive indicators of maternal and perinatal pregnancy outcomes are needed to ameliorate the pregnancy policy for women of childbearing age.

Interpregnancy interval and maternal and neonatal morbidity: a nationwide cohort study

This study aimed to assess the association between interpregnancy interval (IPI)-the time from childbirth to conception of the next pregnancy-and maternal and neonatal morbidity. The World Health Organization (WHO) currently recommends an IPI of at least 24 months after a live birth to reduce adverse birth outcomes. However, assessing the relationship between IPI and perinatal outcome is complicated by confounding factors. We conducted a nationwide population-based cohort study using Swedish registry data, allowing for adjustment of maternal characteristics and health at first birth. The study population consisted of all women with a singleton, live, and vaginal first birth with a second singleton birth within five years during 1997-2017, covering 327,912 women and 655,824 neonates. IPI was grouped into six-month intervals with 24-29 months as the reference. The association between IPI and morbidity was examined using multivariate logistic regression. For women having a vaginal delivery at their first birth, intervals < 24-29 months were associated with decreased maternal morbidity and unaffected neonatal morbidity. Intervals > 24-29 months were associated with increased maternal and neonatal morbidity. Our findings question the relevance of WHO's recommendation of an IPI of at least 24 months in a high-income country.

Effect of short inter-pregnancy interval on perinatal outcomes among pregnant women in North-west Ethiopia: A prospective cohort study

Background

Inter-pregnancy interval (IPI) is the elapse of time between the end of one pregnancy and the conception of another pregnancy, while birth to pregnancy interval, is the time gap between live birth and the conception of the next pregnancy. Hence, this study assessed the effects of short inter-pregnancy intervals on perinatal outcomes among women who gave birth in public health institutions of Assosa zone, North-west Ethiopia.

Methods

An institution-based prospective cohort study was conducted among 456 mothers who visited health facilities for the fourth antenatal care appointment (152 exposed and 304 non-exposed). Women who gave their recent birth with the pregnancy interval of <24 months or/and had an abortion history of <6 months were considered as exposed otherwise non-exposed. Data was collected through face-to-face interviews by using questionnaires and checklists. The collected data was entered using Epi-data and exported to STATA for analysis. A log-binomial regression model was used to identify the effect of short inter-pregnancy intervals on the perinatal outcomes.

Results

The overall incidence of adverse perinatal outcomes is 24%. Mothers who had short inter-pregnancy intervals have two times the risk to develop low birth weight (RR: 2.1, 95%CI: 1.16–3.82), and low Apgar score (RR: 2.1, 95%CI: 1.06–2.69). Similarly, the risk to develop small for gestational age (RR: 2.6, 95% CI: 1.19–7.54), and preterm birth (RR: 3.14, 95%CI: 1.05–4.66) was about 3 times among mothers who had short inter-pregnancy interval compared to mothers who had an optimal inter-pregnancy interval.

Conclusion

Short inter-pregnancy interval increases the risk of low birth weight, preterm birth, small for gestational age, and low Apgar score. Health Policy makers, National health managers and health care providers should work on increasing the awareness of optimal inter-pregnancy intervals and postpartum family planning utilization to reduce the effect of short inter-pregnancy intervals on adverse perinatal outcomes.

Association of Interpregnancy Interval With Adverse Birth Outcomes

IMPORTANCE

Many studies have reported an association of interpregnancy interval (IPI) between 2 consecutive births with adverse birth outcomes in low- and middle-income countries. However, most of these studies ignore the implications of some unmeasured confounders.

OBJECTIVE

To explore the association of IPI with adverse perinatal outcomes.

DESIGN, SETTING, AND PARTICIPANTS

This large-scale cohort study used the Guangdong Provincial Women and Children Health Information System in Guangdong Province, China, to obtain birth data recorded between January 1, 2014, and December 31, 2020. Matched-sibling design was used. The final cohort included first-born and second-born sibling pairs delivered by mothers who were permanent residents of Guangdong Province.

EXPOSURES

The exposure variable was IPI, which was categorized as follows: less than 6, 6 to 11, 12 to 17, 18 to 23, 24 to 29, 30 to 35, and 36 or more months.

MAIN OUTCOMES AND MEASURES

The outcome variables were adverse birth outcomes: preterm birth (PTB, gestational age <37 weeks), low birth weight (LBW, <2500 g), and small for gestational age (SGA). Adjusted odds ratio (OR) and interaction odds ratio (IOR) associated with IPI were calculated.

RESULTS

The study consisted of 725 392 sibling pairs of multiparous mothers. Among these mothers, 718 111 (99.0%) were aged 20 to 34 years, and 715 583 (98.7%) were of Han Chinese ethnicity. Unmatched analysis showed that a short IPI of less than 6 months was associated with higher risks of PTB (adjusted OR, 1.96; 95% CI, 1.87-2.06), LBW (adjusted OR, 1.88; 95% CI, 1.79-1.98), and SGA (adjusted OR, 1.34; 95% CI, 1.30-1.38) compared with an IPI of 18 to 23 months. These associations were attenuated in the matched-sibling analysis. An association of short IPI (<6 months) with PTB (adjusted IOR, 1.40; 95% CI, 1.30-1.51), LBW (adjusted IOR, 1.30; 95% CI, 1.21-1.40), and SGA (adjusted IOR, 1.16; 95% CI, 1.11-1.22) remained in the matched analysis. For IPI of 36 months or more, the odds of PTB (adjusted OR, 1.08; 95% CI, 1.03-1.14) and LBW (adjusted OR, 1.13; 95% CI, 1.07-1.19) in the unmatched analysis were also greater than the reference interval (18-23 months), but not for SGA (adjusted OR, 0.96; 95% CI, 0.93-0.99). Associations between a long IPI (≥36 months) and PTB (adjusted IOR, 1.10; 95% CI, 1.02-1.19) and LBW (adjusted IOR, 1.16; 95% CI, 1.07-1.26) remained through the sibling comparisons.

CONCLUSIONS AND RELEVANCE

Results of this study indicated that mothers with a short (<6 months) or long (≥36 months) IPI had greater odds of adverse birth outcomes. The findings may inform family planning policies and guide individuals and families who are planning for another pregnancy in China.

Using probabilistic record linkage and propensity-score matching to identify a community-based comparison population

In retrospective cohort studies of interventions disseminated to communities, it is challenging to find comparison groups with high-quality data for evaluation. We present one methodological approach as part of our study of birth outcomes of second-born children in a home visiting (HV) program targeting first-time mothers. We used probabilistic record linkage to link Connecticut's Nurturing Families Network (NFN) HV program and birth-certificate data for children born from 2005 to 2015. We identified two potential comparison groups: a propensity-score-matched group from the remaining birth certificate sample and eligible-but-unenrolled families. An analysis of interpregnancy interval (IPI) is presented to exemplify the approach. We identified the birth certificates of 4822 NFN families. The propensity-score-matched group had 14,219 families (3-to-1 matching) and we identified 1101 eligible-but-unenrolled families. Covariates were well balanced for the propensity-score-matched group, but poorly balanced for the eligible-but-unenrolled group. No program effect on IPI was found. By combining propensity-score matching and probabilistic record linkage, we were able to retrospectively identify relatively large comparison groups for quasi-experimental research. Using birth certificate data, we accessed outcomes for all of these individuals from a single data source. Multiple comparison groups allow us to confirm findings when each method has some limitations. Other researchers seeking community-based comparison groups could consider a similar approach.

Is there an optimal inter-delivery interval in women who underwent trial of labor after cesarean delivery (TOLAC)?

BACKGROUND

Inter-delivery interval (IDI) has been proven to be a factor associated with adverse maternal and neonatal outcomes. However, the optimal IDI in trial of labor after cesarean delivery (TOLAC) remains unclear. We aimed to investigate the association between IDI and major maternal and neonatal outcomes in women who underwent TOLAC.

METHODS

A multicenter, retrospective cohort study including five hospitals was conducted between January 2018 and December 2019 in Foshan, China. This study included 1080 pregnant women with one or two cesarean deliveries who attempted a TOLAC. Data on maternal and neonatal outcomes were collected from the electronic record system. Maternal and neonatal outcomes in different groups of IDI were compared by univariate and multivariable analyses.

RESULTS

A short IDI of < 24 months did not show a statistically significant association with uterine rupture in the univariate analysis (P = 0.668). In multivariable analysis, the incidences of postpartum hemorrhage (OR 19.6, 95% CI:4.4-90.9, P < 0.05), preterm birth (OR 5.5, 95% CI:1.5-21.3, P < 0.05), and low birth weight (OR 3.5, 95% CI:1.2-10.3, P < 0.05) were significantly increased in women with an IDI of < 24 months than in those with a normal interval (24-59 months). Infection morbidity (OR 1.8, 95% CI:1.4-7.9, P < 0.05), transfusion (OR 7.4, 95% CI:1.4-40.0, P < 0.05), and neonatal unit admission (OR 2.6, 95% CI:1.4-5.0, P < 0.05) were significantly increased in women with an IDI of 120 months or more than in those with a normal interval. Postpartum hemorrhage (P = 0.062) had a trend similar to that of a significant IDI of 120 months or more. We found no statistically significant difference in maternal and neonatal outcomes between 24-59 months and 60-119 months.

CONCLUSIONS

An IDI of less than 24 months or 120 months or more increased the risk of major maternal and neonatal outcomes. We recommend that the optimal interval for women who underwent TOLAC should be 24 to 119 months.

Interpregnancy intervals and adverse birth outcomes in high-income countries: An international cohort study

BACKGROUND

Most evidence for interpregnancy interval (IPI) and adverse birth outcomes come from studies that are prone to incomplete control for confounders that vary between women. Comparing pregnancies to the same women can address this issue.

METHODS

We conducted an international longitudinal cohort study of 5,521,211 births to 3,849,193 women from Australia (1980-2016), Finland (1987-2017), Norway (1980-2016) and the United States (California) (1991-2012). IPI was calculated based on the time difference between two dates-the date of birth of the first pregnancy and the date of conception of the next (index) pregnancy. We estimated associations between IPI and preterm birth (PTB), spontaneous PTB, and small-for-gestational age births (SGA) using logistic regression (between-women analyses). We also used conditional logistic regression comparing IPIs and birth outcomes in the same women (within-women analyses). Random effects meta-analysis was used to calculate pooled adjusted odds ratios (aOR).

RESULTS

Compared to an IPI of 18-23 months, there was insufficient evidence for an association between IPI <6 months and overall PTB (aOR 1.08, 95% CI 0.99-1.18) and SGA (aOR 0.99, 95% CI 0.81-1.19), but increased odds of spontaneous PTB (aOR 1.38, 95% CI 1.21-1.57) in the within-women analysis. We observed elevated odds of all birth outcomes associated with IPI ≥60 months. In comparison, between-women analyses showed elevated odds of adverse birth outcomes for <12 month and >24 month IPIs.

CONCLUSIONS

We found consistently elevated odds of adverse birth outcomes following long IPIs. IPI shorter than 6 months were associated with elevated risk of spontaneous PTB, but there was insufficient evidence for increased risk of other adverse birth outcomes. Current recommendations of waiting at least 24 months to conceive after a previous pregnancy, may be unnecessarily long in high-income countries.

Effects of previous birth interval length on child outcomes can be estimated in a sibling analysis even when there are only two siblings

BACKGROUND

There is much interest in how the length of the previous birth interval affects various child outcomes, and it has become increasingly common to estimate such effects from sibling models. This is because one then controls for unobserved determinants of the outcome that are shared between the siblings and linked to the birth interval length. However, it is a common idea that such effects can only be estimated from data on families with three or more children.

OBJECTIVE

The goal of this paper is to show, through an intuitive argument and a simulation experiment, that it is possible to estimate effects of birth interval only from families with two children.

METHODS

Observations are simulated from two equations for fertility and one equation for child mortality. The fertility equations include a random term that is assumed to be correlated with the random term in the mortality equation. Mortality models are then estimated from the simulated observations. This is done 1000 times, and the averages of the 1000 sets of estimates are calculated.

RESULTS

The simulation experiment illustrates that it is indeed possible (by using a model specification that takes into account that no birth interval is defined for the first birth) to estimate birth interval effects in sibling models even when the data include only families with two children.

CONCLUSION

It is good news that families with only two children can contribute to the estimation of birth interval effects. This is because, with a broader basis for the estimation, the precision is improved and there is less reason for concern about the general relevance of the estimates. An important limitation, however, is that it is potentially problematic to control for maternal age in a sibling model estimated only for the first and second child.

Interpregnancy interval and hypertensive disorders of pregnancy: A population-based cohort study

BACKGROUND

Despite extensive research on risk factors and mechanisms, the extent to which interpregnancy interval (IPI) affects hypertensive disorders of pregnancy in high-income countries remains unclear.

OBJECTIVES

To examine the association between IPI and hypertensive disorders of pregnancy in a high-income country setting using both within-mother and between-mother comparisons.

METHODS

A retrospective population-based cohort study was conducted among 103 909 women who delivered three or more consecutive singleton births (n = 358 046) between 1980 and 2015 in Western Australia. We used conditional Poisson regression with robust variance, matching intervals of the same mother and adjusted for factors that vary within-mother across pregnancies, to investigate the association between IPI categories (reference 18-23 months), and the risk of hypertensive disorders of pregnancy. For comparison with previous studies, we also applied unmatched Poisson regression (between-mother analysis).

RESULTS

The incidence of preeclampsia and gestational hypertension during the study period was 4%, and 2%, respectively. For the between-mother comparison, mothers with intervals of 6-11 months had lower risk of preeclampsia with adjusted relative risk (RR) 0.92 (95% confidence interval [CI] 0.85, 0.98) compared to reference category of 18-23 months. With the within-mother matched design, we estimated a larger effect of long IPI on risk of preeclampsia (RR 1.29, 95% CI 1.18, 1.42 for 60-119 months; and RR 1.30, 95% CI 1.10, 1.53 for intervals ≥120 months) compared to 18-23 months. Short IPIs were not associated with hypertensive disorders of pregnancy.

CONCLUSIONS

In our cohort, longer IPIs were associated with increased risk of preeclampsia. However, there was insufficient evidence to suggest that short IPIs (<6 months) increase the risks of hypertensive disorders of pregnancy.

Associations between interpregnancy interval and preterm birth by previous preterm birth status in four high-income countries: a cohort study

OBJECTIVE

To investigate the effect of interpregnancy interval (IPI) on preterm birth (PTB) according to whether the previous birth was preterm or term.

DESIGN

Cohort study.

SETTING

USA (California), Australia, Finland, Norway (1980-2017).

POPULATION

Women who gave birth to first and second (n = 3 213 855) singleton livebirths.

METHODS

Odds ratios (ORs) for PTB according to IPIs were modelled using logistic regression with prognostic score stratification for potential confounders. Within-site ORs were pooled by random effects meta-analysis.

OUTCOME MEASURE

PTB (gestational age <37 weeks).

RESULTS

Absolute risk of PTB for each IPI was 3-6% after a previous term birth and 17-22% after previous PTB. ORs for PTB differed between previous term and preterm births in all countries (P-for-interaction ≤ 0.001). For women with a previous term birth, pooled ORs were increased for IPI <6 months (OR 1.50, 95% CI 1.43-1.58); 6-11 months (OR 1.10, 95% CI 1.04-1.16); 24-59 months (OR 1.16, 95% CI 1.13-1.18); and ≥ 60 months (OR 1.72, 95%CI 1.60-1.86), compared with 18-23 months. For previous PTB, ORs were increased for <6 months (OR 1.30, 95% CI 1.18-1.42) and ≥60 months (OR 1.29, 95% CI 1.17-1.42), but were less than ORs among women with a previous term birth (P < 0.05).

CONCLUSIONS

Associations between IPI and PTB are modified by whether or not the previous pregnancy was preterm. ORs for short and long IPIs were higher among women with a previous term birth than a previous PTB, which for short IPI is consistent with the maternal depletion hypothesis. Given the high risk of recurrence and assuming a causal association between IPI and PTB, IPI remains a potentially modifiable risk factor for women with previous PTB.

TWEETABLE ABSTRACT

Short versus long interpregnancy intervals associated with higher ORs for preterm birth (PTB) after a previous PTB.

Profile of severely growth-restricted births undelivered at 40 weeks in Western Australia

PURPOSE

To investigate the proportion of severely growth-restricted singleton births < 3rd percentile (proxy for severe fetal growth restriction; FGR) undelivered at 40 weeks (FGR_40), and compare maternal characteristics and outcomes of FGR_40 births and FGR births at 37-39 weeks' (FGR_37-39) to those not born small-for-gestational-age at term (Not SGA_37+).

METHODS

The annual rates of singleton FGR_40 births from 2006 to 2015 were calculated using data from linked Western Australian population health datasets. Using 2013-2015 data, maternal factors associated with FGR births were investigated using multinomial logistic regression to estimate odds ratios (OR) with 95% confidence intervals (CI) while relative risks (RR) of birth outcomes between each group were calculated using Poisson regression. Neonatal adverse outcomes were identified using a published composite indicator (diagnoses, procedures and other factors).

RESULTS

The rate of singleton FGR_40 births decreased by 23.0% between 2006 and 2015. Factors strongly associated with FGR_40 and FGR_37-39 births compared to Not SGA_37+ births included the mother being primiparous (ORs 3.13: 95% CI 2.59-3.79; 1.69, 95% CI 1.47, 1.94, respectively) and ante-natal smoking (ORs 2.55, 95% CI 1.97, 3.32; 4.48, 95% CI 3.74, 5.36, respectively). FGR_40 and FGR_37-39 infants were more likely to have a neonatal adverse outcome (RRs 1.70, 95% CI 1.41, 2.06 and 2.46 95% CI 2.18, 2.46, respectively) compared to Not SGA 37+ infants.

CONCLUSIONS

Higher levels of poor perinatal outcomes among FGR births highlight the importance of appropriate management including fetal growth monitoring. Regular population-level monitoring of FGR_40 rates may lead to reduced numbers of poor outcomes.

Beyond birth outcomes: Interpregnancy interval and injury-related infant mortality

BACKGROUND

Several studies have examined the association between IPI and birth outcomes, but few have explored the association between interpregnancy interval (IPI) and postnatal outcomes.

OBJECTIVE

We examined the association between IPI and injury-related infant mortality, a leading cause of postneonatal mortality.

METHODS

We used 2011-2015 US period-linked birth-infant death vital statistics data to generate a multiyear birth cohort of non-first-born singleton births (N = 9 782 029). IPI was defined as the number of months between a live birth and the start of the pregnancy leading to the next live birth. Causes of death in the first year of life were identified using ICD-10 codes. Hazard ratios (HR) for IPI categories were estimated using Cox proportional hazards models adjusted for birth order, county poverty level, and maternal characteristics (marital status, race/ethnicity, education, age at previous birth).

RESULTS

After adjustment, overall infant mortality (48.1 per 10 000 births) was higher for short and long IPIs compared with IPI 18-23 months (reference): <6, aHR 1.61, 95% CI 1.54, 1.68; 6-11, aHR 1.22, 95% CI 1.17, 1.26; and 60+ months, aHR 1.12, 95% CI 1.08, 1.16. In comparison, the risk of injury-related infant mortality (4.4 per 10 000 births) decreased with longer IPIs: <6, aHR 1.77, 95% CI 1.55, 2.01; 6-11, aHR 1.41, 95% CI 1.25, 1.59; 12-17, aHR 1.25, 95% CI 1.10, 1.41; 24-59, aHR 0.78, 95% CI 0.69, 0.87; and 60+ months, aHR 0.55, 95% CI 0.48, 0.62.

CONCLUSION

Unlike overall infant mortality, injury-related infant mortality decreased with IPI length. While injury-related deaths are rare, these patterns suggest that the timing between births may be a marker of risk for fatal infant injuries. The first year postpartum may be an ideal time for the delivery of evidence-based injury prevention programmes as well as family planning services.

Developing evidence-based recommendations for optimal interpregnancy intervals in high-income countries: protocol for an international cohort study

INTRODUCTION

Short interpregnancy interval (IPI) has been linked to adverse pregnancy outcomes. WHO recommends waiting at least 2 years after a live birth and 6 months after miscarriage or induced termination before conception of another pregnancy. The evidence underpinning these recommendations largely relies on data from low/middle-income countries. Furthermore, recent epidemiological investigations have suggested that these studies may overestimate the effects of IPI due to residual confounding. Future investigations of IPI effects in high-income countries drawing from large, population-based data sources are needed to inform IPI recommendations. We aim to assess the impact of IPIs on maternal and child health outcomes in high-income countries.

METHODS AND ANALYSIS

This international longitudinal retrospective cohort study will include more than 18 million pregnancies, making it the largest study to investigate IPI in high-income countries. Population-based data from Australia, Finland, Norway and USA will be used. Birth records in each country will be used to identify consecutive pregnancies. Exact dates of birth and clinical best estimates of gestational length will be used to estimate IPI. Administrative birth and health data sources with >99% coverage in each country will be used to identify maternal sociodemographics, pregnancy complications, details of labour and delivery, birth and child health information. We will use matched and unmatched regression models to investigate the impact of IPI on maternal and infant outcomes, and conduct meta-analysis to pool results across countries.

ETHICS AND DISSEMINATION

Ethics boards at participating sites approved this research (approval was not required in Finland). Findings will be published in peer-reviewed journals and presented at international conferences, and will inform recommendations for optimal IPI in high-income countries. Findings will provide important information for women and families planning future pregnancies and for clinicians providing prenatal care and giving guidance on family planning.

Invited Commentary: Promise and Pitfalls of the Sibling Comparison Design in Studies of Optimal Birth Spacing

Numerous observational studies have shown that infants born after short interpregnancy intervals (the interval between birth and subsequent conception) are more likely to experience adverse perinatal outcomes than infants born following longer intervals. Yet it remains controversial whether the link between short interpregnancy interval and adverse outcomes is causal or is confounded by factors such as low socioeconomic position, inadequate access to health care, and unintended pregnancy. Sibling comparison studies, which use a woman as her own control by comparing exposure and outcome status of her different pregnancies (i.e., comparing sibling offspring), have gained popularity as a strategy to reduce confounding by these difficult-to-measure factors that are nevertheless relatively stable within women. A variant of this approach, used by Regan et al. (Am J Epidemiol. 2019;188(1):9-16) and reported in this issue of the Journal, is a maternally matched design based on a single interpregnancy interval per woman. Using real and simulated data, we highlight underappreciated shortcomings of these designs that could limit the validity of study findings. In particular, we illustrate how the single-interval variant appears to derive estimates from comparisons between different mothers, not within mothers. Future studies of optimal birth spacing using sibling comparison designs should examine in detail the potential consequences of these methodological limitations.