The Causal Inference Framework: A Primer on Concepts and Methods for Improving the Study of Well-Woman Childbearing Processes

Impaired oxygenation after lung resection: Incidence and perioperative risk factors

STUDY OBJECTIVE

To estimate the incidence of postoperative oxygenation impairment after lung resection in the era of lung-protective management, and to identify perioperative factors associated with that impairment.

DESIGN

Registry-based retrospective cohort study.

SETTING

Two large academic hospitals in the United States.

PATIENTS

3081 ASA I-IV patients undergoing lung resection.

MEASUREMENTS

79 pre- and intraoperative variables, selected for inclusion based on a causal inference framework. The primary outcome of impaired oxygenation, an early marker of lung injury, was defined as at least one of the following within seven postoperative days: (1) SpO2 < 92%; (2) imputed PaO2/FiO2 < 300 mmHg [(1) or (2) occurring at least twice within 24 h]; (3) intensive oxygen therapy (mechanical ventilation or > 50% oxygen or high-flow oxygen).

MAIN RESULTS

Oxygenation was impaired within seven postoperative days in 70.8% of patients (26.6% with PaO2/FiO2 < 200 mmHg or intensive oxygen therapy). In multivariable analysis, each additional cmH2O of intraoperative median driving pressure was associated with a 7% higher risk of impaired oxygenation (OR 1.07; 95%CI 1.04 to 1.10). Higher median intraoperative FiO2 (OR 1.23; 95%CI 1.14 to 1.31 per 0.1) and PEEP (OR 1.12; 95%CI 1.04 to 1.21 per 1 cm H2O) were also associated with increased risk. History of COPD (OR 2.55; 95%CI 1.95 to 3.35) and intraoperative albuterol administration (OR 2.07; 95%CI 1.17 to 3.67) also showed reliable effects.

CONCLUSIONS

Impaired postoperative oxygenation is common after lung resection and is associated with potentially modifiable pre- and intraoperative respiratory factors.

Is Oxytocin Administration During Childbirth Associated With Increased Risk for Postpartum Posttraumatic Stress Symptoms?: A Preliminary Investigation

INTRODUCTION

Synthetic oxytocin is one of the most regularly administered medications to facilitate labor induction and augmentation. The present study examined the associations between oxytocin administration during childbirth and postpartum posttraumatic stress symptoms (PTSS).

MATERIALS AND METHODS

In a multicenter longitudinal study, women completed questionnaires during pregnancy and at 2 months postpartum ( N = 386). PTSS were assessed with the Impact of Event Scale. Logistic regression was used to examine the difference in PTSS at Time 2 between women who received oxytocin and women who did not.

RESULTS

In comparison with women who did not receive oxytocin, women who received oxytocin induction were 3.20 times as likely to report substantial PTSS ( P = .036, 95% confidence interval: 1.08-9.52), and women who received oxytocin augmentation were 3.29 times as likely to report substantial PTSS ( P = .036, 95% confidence interval: 1.08-10.03), after controlling for being primiparous, preeclampsia, prior mental health diagnosis, mode of birth, postpartum hemorrhage, and satisfaction with staff.

DISCUSSION

Oxytocin administration was associated with a 3-fold increased risk of PTSS. The findings may reflect biological and psychological mechanisms related to postpartum mental health and call for future research to establish the causation of this relationship.

Approaches to estimate bidirectional causal effects using Mendelian randomization with application to body mass index and fasting glucose

Mendelian randomization (MR) is an epidemiological framework using genetic variants as instrumental variables (IVs) to examine the causal effect of exposures on outcomes. Statistical methods based on unidirectional MR (UMR) are widely used to estimate the causal effects of exposures on outcomes in observational studies. To estimate the bidirectional causal effects between two phenotypes, investigators have naively applied UMR methods separately in each direction. However, bidirectional causal effects between two phenotypes create a feedback loop that biases the estimation when UMR methods are naively applied. To overcome this limitation, we proposed two novel approaches to estimate bidirectional causal effects using MR: BiRatio and BiLIML, which are extensions of the standard ratio, and limited information maximum likelihood (LIML) methods, respectively. We compared the performance of the two proposed methods with the naive application of UMR methods through extensive simulations of several scenarios involving varying numbers of strong and weak IVs. Our simulation results showed that when multiple strong IVs are used, the proposed methods provided accurate bidirectional causal effect estimation in terms of median absolute bias and relative median absolute bias. Furthermore, compared to the BiRatio method, the BiLIML method provided a more accurate estimation of causal effects when weak IVs were used. Therefore, based on our simulations, we concluded that the BiLIML should be used for bidirectional causal effect estimation. We applied the proposed methods to investigate the potential bidirectional relationship between obesity and diabetes using the data from the Multi-Ethnic Study of Atherosclerosis cohort. We used body mass index (BMI) and fasting glucose (FG) as measures of obesity and type 2 diabetes, respectively. Our results from the BiLIML method revealed the bidirectional causal relationship between BMI and FG in across all racial populations. Specifically, in the White/Caucasian population, a 1 kg/m2 increase in BMI increased FG by 0.70 mg/dL (95% confidence interval [CI]: 0.3517-1.0489; p = 8.43×10-5), and 1 mg/dL increase in FG increased BMI by 0.10 kg/m2 (95% CI: 0.0441-0.1640; p = 6.79×10-4). Our study provides novel findings and quantifies the effect sizes of the bidirectional causal relationship between BMI and FG. However, further studies are needed to understand the biological and functional mechanisms underlying the bidirectional pathway.

An alternative model of maternity care for low-risk birth: Maternal and neonatal outcomes utilizing the midwifery-based birth center model

OBJECTIVE

To assess key birth outcomes in an alternative maternity care model, midwifery-based birth center care.

DATA SOURCES

The American Association of Birth Centers Perinatal Data Registry and birth certificate files, using national data collected from 2009 to 2019.

STUDY DESIGN

This observational cohort study compared key clinical birth outcomes of women at low risk for perinatal complications, comparing those who received care in the midwifery-based birth center model versus hospital-based usual care. Linear regression analysis was used to assess key clinical outcomes in the midwifery-based group as compared with hospital-based usual care. The hospital-based group was selected using nearest neighbor matching, and the primary linear regressions were weighted using propensity score weights (PSWs). The key clinical outcomes considered were cesarean delivery, low birth weight, neonatal intensive care unit admission, breastfeeding, and neonatal death. We performed sensitivity analyses using inverse probability weights and entropy balancing weights. We also assessed the remaining role of omitted variable bias using a bounding methodology.

DATA COLLECTION

Women aged 16-45 with low-risk pregnancies, defined as a singleton fetus and no record of hypertension or cesarean section, were included. The sample was selected for records that overlapped in each year and state. Counties were included if there were at least 50 midwifery-based birth center births and 300 total births. After matching, the sample size of the birth center cohort was 85,842 and the hospital-based cohort was 261,439.

PRINCIPAL FINDINGS

Women receiving midwifery-based birth center care experienced lower rates of cesarean section (-12.2 percentage points, p < 0.001), low birth weight (-3.2 percentage points, p < 0.001), NICU admission (-5.5 percentage points, p < 0.001), neonatal death (-0.1 percentage points, p < 0.001), and higher rates of breastfeeding (9.3 percentage points, p < 0.001).

CONCLUSIONS

This analysis supports midwifery-based birth center care as a high-quality model that delivers optimal outcomes for low-risk maternal/newborn dyads.

Maternal Morbidity Predicted by an Intersectional Social Determinants of Health Phenotype: A Secondary Analysis of the NuMoM2b Dataset

Maternal race, ethnicity and socio-economic position are known to be associated with increased risk for a range of poor pregnancy outcomes, including maternal morbidity and mortality. Previously, researchers seeking to identify the contributing factors focused on maternal behaviors and pregnancy complications. Less understood is the contribution of the social determinants of health (SDoH) in observed differences by race/ethnicity in these key outcomes. In this secondary analysis of the Nulliparous Pregnancy Outcomes Study: Monitoring Mothers-to-Be (nuMoM2b) dataset, latent mixture modeling was used to construct groups of healthy, nulliparous participants with a non-anomalous fetus in a cephalic presentation having a trial of labor (N = 5763) based on SDoH variables. The primary outcome was a composite score of postpartum maternal morbidity. A postpartum maternal morbidity event was experienced by 350 individuals (6.1%). Latent class analysis using SDoH variables revealed six groups of participants, with postpartum maternal morbidity rates ranging from 8.7% to 4.5% across groups (p < 0.001). Two SDoH groups had the highest odds for maternal morbidity. These higher-risk groups were comprised of participants with the lowest income and highest stress and those who had lived in the USA for the shortest periods of time. SDoH phenotype predicted MM outcomes and identified two important, yet distinct groups of pregnant people who were the most likely have a maternal morbidity event.

Association between first and second stage of labour duration and mode of delivery: A population-based cohort study

BACKGROUND

Active first stage of labour duration can widely vary between women. However, the nature of the relationship between the active first stage and second stage of labour duration is sparsely studied.

OBJECTIVES

To determine whether active first stage of labour duration (i) influences second stage of labour duration; and (ii) is associated with mode of delivery.

METHODS

A population-based cohort study of 13,379 women primiparous women, with spontaneous start in Stockholm-Gotland Region, Sweden, between 2008 and 2014. Duration of the active first stage of labour was examined in relation to second-stage duration using univariate and multivariable quantile regressions, with the first quartile (first stage duration) as the reference. Nonlinearity of associations was tested by restricted cubic splines. Association between active first-stage duration with mode of delivery was estimated using a multinomial logistic regression based on adjusted odds ratios.

RESULTS

Longer active first stage of labour duration was linearly associated with longer second stage of labour duration until approximately 12 h of active first stage of labour duration. After 12 h, a non-linear trend is seen, demonstrated by a plateau in the second-stage duration. In addition, longer active first stage of labour duration was associated with increased occurrence of operative vaginal delivery (adjusted odds ratio 3.36, 95% confidence interval [CI] 2.89, 3.89) and caesarean delivery (adjusted odds ratio 4.75, 95% CI 3.85, 5.80).

CONCLUSIONS

Among primiparous women with spontaneous onset of labour, longer active first stage of labour duration was associated with both longer second stage of labour duration and higher odds of operative delivery. This study contributes with findings, which may inform future discussions regarding how to properly account for second-stage duration, with applications in obstetric and perinatal epidemiology.

Association of body mass index and maternal age with first stage duration of labour

To evaluate associations between early-pregnancy body mass index (BMI) and active first stage labour duration, accounting for possible interaction with maternal age, we conducted a cohort study of women with spontaneous onset of labour allocated to Robson group 1. Quantile regression analysis was performed to estimate first stage labour duration between BMI categories in two maternal age subgroups (more and less than 30 years). Results show that obesity (BMI > 30) among younger women (< 30 years) increased the median labour duration of first stage by 30 min compared with normal weight women (BMI < 25), and time difference estimated at the 90th quantile was more than 1 h. Active first stage labour time differences between obese and normal weight women was modified by maternal age. In conclusion: (a) obesity is associated with longer duration of first stage of labour, and (b) maternal age is an effect modifier for this association. This novel finding of an effect modification between BMI and maternal age contributes to the body of evidence that supports a more individualized approach when describing labour duration.

The association between longer durations of the latent phase of labor and subsequent perinatal processes and outcomes among midwifery patients

BACKGROUND/OBJECTIVE

To evaluate the association between the duration of the latent phase of labor and subsequent processes and outcomes.

METHODS

Secondary analysis of prospectively collected data among 1,189 women with low-risk pregnancies and spontaneous labor.

RESULTS

Longer latent phase duration was associated with labor dystocia (eg, nulliparous ≥ mean [compared with < mean] aOR 3.95 [2.70-5.79]; multiparous ≥ mean [compared with < mean] aOR 5.45 [3.43-8.65]), interventions to ameliorate dystocia, and epidurals to cope or rest (eg, oxytocin augmentation: nulliparous > 80th% [compared with < 80th%] aOR 6.39 [4.04-10.12]; multiparous ≥ 80th% [compared with < 80th%] aOR 6.35 [3.79-10.64]). Longer latent phase duration was also associated with longer active phase and second stage. There were no associations between latent phase duration and risk for cesarean delivery or postpartum hemorrhage in a practice setting with relatively low rates of primary cesarean. Newborns born to multiparous women with latent phase of labor durations at and beyond the 80th% were more frequently admitted to the NICU (≥80th% [compared with < 80th%] aOR 2.7 [1.22-5.84]); however, two-thirds of these NICU admissions were likely for observation only.

CONCLUSIONS

Longer duration of the spontaneous latent phase of labor among women with low-risk pregnancies may signal longer total labor processes, leading to an increase in diagnosis of dystocia, interventions to manage dystocia, and epidural use. Apart from multiparous neonatal NICU admission, no other maternal or child morbidity outcomes were elevated with longer duration of the latent phase of labor.

Predicting Postpartum Hemorrhage After Low-Risk Vaginal Birth by Labor Characteristics and Oxytocin Administration

OBJECTIVE

To determine the odds of postpartum hemorrhage (PPH) in low-risk women who gave birth vaginally and were exposed to different durations and dosages of oxytocin across a range of labor durations during spontaneous or induced labor.

DESIGN

A retrospective cross-sectional analysis of data from the Consortium for Safe Labor.

SETTING

Data were gathered from 12 clinical institutions across the United States from 2002 to 2008.

PARTICIPANTS

After exclusion of high-risk conditions associated with PPH, we examined data from 27,072 women who gave birth vaginally.

METHODS

PPH was defined as estimated blood loss of greater than 500 ml at the time of birth and/or a diagnostic code for PPH before hospital discharge. We included covariates were if they were associated with oxytocin use and PPH and did not mediate oxytocin use. We used regression models to determine the likelihood of PPH overall and within the induced and spontaneous labor groups separately. We used subgroup analyses within specific durations of labor to clarify the findings.

RESULTS

The overall rate of PPH was 3.9%. Women with induced labor experienced PPH more frequently than women who labored spontaneously. Labor augmentation was associated with greater adjusted odds for PPH when oxytocin was infused for more than 4 hours. Longer duration of spontaneous labor and the second stage of labor did not change this association. Oxytocin use during labor induction increased the odds for PPH when administered for more than 7 hours. The odds further increased when induction lasted longer than 12 hours and/or the second stage of labor was longer than 3 hours.

CONCLUSION

Strategies for judicious oxytocin administration may help mitigate PPH in low-risk women having vaginal birth.

Influence of midwifery presence in United States centers on labor care and outcomes of low-risk parous women: A Consortium on Safe Labor study

BACKGROUND

Sixty percent of United States births are to multiparous women. Hospital-level policies and culture may influence intrapartum care and birth outcomes for this large population, yet have been poorly explored using a large, diverse sample. We sought to use national United States data to analyze the association between midwifery presence in maternity care teams and the birth processes and outcomes of low-risk parous women.

METHODS

We conducted a retrospective cohort study using Consortium on Safe Labor data from low-risk parous women in either interprofessional care (n = 12 125) or noninterprofessional care centers (n = 8996). Unadjusted, adjusted (age, race, health insurance type), propensity-adjusted, and propensity-matched logistic regression models were used to assess processes and outcomes.

RESULTS

There was concordance in outcome differences across regression models. With propensity score matching, women at interprofessional centers, compared with women at noninterprofessional centers, were 85% less likely to have labor induced (risk ratio [RR] 0.15; 95% CI 0.14-0.17). The risk for primary cesarean birth among low-risk parous women was 36% lower at interprofessional centers (RR 0.64; 95% CI 00.52-0.79), whereas the likelihood of vaginal birth after cesarean for this population was 31% higher (RR 1.31; 95% CI 1.10-1.56). There were no significant differences in neonatal outcomes.

CONCLUSIONS

Parous women have significantly higher rates of vaginal birth, including vaginal birth after cesarean, and lower likelihood of labor induction when cared for in centers with midwives. Our findings are consistent with smaller analyses of midwifery practice and support integrated, team-based models of perinatal care to improve maternal outcomes.

Cost-Effectiveness of Continuous Support From a Layperson During a Woman's First Two Births

OBJECTIVE

To evaluate the cost-effectiveness and health outcomes related to continuous support from a layperson during a woman's first two births in a theoretical population.

DESIGN

Cost-effectiveness analysis.

PARTICIPANTS

A theoretical cohort of 1.2 million women based on an approximation of annual low-risk, nulliparous, term, singleton births in the United States with the assumption that these women have second births. This reflects the average number of births per woman in the United States.

METHODS

We designed a cost-effectiveness model to compare outcomes in women with continuous support from relatives, friends, or community members with minimal to no training (excluding trained doulas) during labor and birth compared with outcomes for women with no continuous support. Outcomes included mode of birth, uterine rupture, hysterectomy, maternal death, cost, and quality-adjusted life years (QALYs). We derived probabilities from the literature and set a cost-effectiveness threshold at $100,000/QALY.

RESULTS

In this theoretical model, continuous support by a layperson during the first birth resulted in fewer cesarean births, decreased costs, and increased QALYs for the first and subsequent births. Women with support from laypersons had 71,090 fewer cesarean births, 35 fewer uterine ruptures, 9 fewer hysterectomies, and 16 fewer maternal deaths, which saved $364 million with 2,673 increased QALYs. Sensitivity analyses showed that continuous support in the first birth was cost-effective even when varying the estimate of lost wages of the support person up to $708.

CONCLUSION

Continuous labor support from a layperson leads to fewer cesarean births, improved outcomes, decreased costs, and increased QALYs. This highlights the need to increase women's access to continuous layperson support during labor and birth uninhibited by financial and institutional barriers.