The use of customised versus population-based birthweight standards in predicting perinatal mortality

Customized birth-weight centiles and placenta-related fetal growth restriction

OBJECTIVE

The value of using customized birth-weight centiles to improve the diagnostic accuracy for fetal growth restriction (FGR), in comparison with using population-based charts, remains a matter of debate. One potential explanation for the conflicting data is that most studies used measures of perinatal mortality and morbidity as proxies for placenta-mediated FGR, many of which are not specific and may be confounded by other factors such as prematurity. The aim of this study was to compare the diagnostic accuracy of small-for-gestational age (SGA) at birth, defined according to customized vs population-based charts, for associated abnormal placental pathology.

METHODS

This was a secondary analysis of data from a prospective cohort study on risk factors for placenta-mediated complications and abnormal placental pathology in low-risk nulliparous women. All placentae were sent for detailed histopathological examination by two perinatal pathologists. The primary exposure was SGA, defined as birth weight < 10^th centile for gestational age using either a customized (SGA_cust ) or a population-based (SGA_pop ) birth-weight reference. The outcomes of interest were one of three types of abnormal placental pathology associated with FGR: maternal vascular malperfusion (MVM), chronic villitis and fetal vascular malperfusion (FVM). Adjusted relative risks (aRR) with 95% CIs were estimated using modified Poisson regression analysis, with adjustment for smoking, body mass index and aspirin treatment.

RESULTS

A total of 857 nulliparous women met the study criteria. The proportions of infants identified as SGA based on the customized and population-based charts were 12.6% (108/857) and 11.4% (98/857), respectively. A diagnosis of SGA using either customized or population-based charts was associated with an increased risk of any placental pathology (aRR, 3.04 (95% CI, 2.29-4.04) and 1.60 (95% CI, 1.10-2.31), respectively) and MVM pathology (aRR, 12.33 (95% CI, 6.60-23.03) and 5.29 (95% CI, 2.87-9.76), respectively). SGA_cust , but not SGA_pop , was also associated with an increased risk for chronic villitis (aRR, 1.85 (95% CI, 1.07-3.18)) and FVM pathology (aRR, 2.48 (95% CI, 1.25-4.93)). SGA_cust had a higher detection rate for any placental pathology (30.3% vs 17.1%; P < 0.001), MVM pathology (63.2% vs 39.5%; P = 0.003) and chronic villitis (20.8% vs 8.3%; P = 0.007) than did SGA_pop , for a similar false-positive rate. This was mainly the result of a higher detection rate for abnormal pathology in the white and East-Asian subgroups and a lower false-positive rate for abnormal pathology in the South-Asian subgroup by SGA_cust than by SGA_pop . In addition, pregnancies in the SGA_cust group, but not those in the SGA_pop group, were more likely to be complicated by preterm birth and a low 5-min Apgar score than were the corresponding non-SGA group.

CONCLUSION

These findings suggest that customized birth-weight centiles may be superior to population-based birth-weight centiles in detecting FGR that is due to underlying placental disease. © 2020 International Society of Ultrasound in Obstetrics and Gynecology.

Prediction of adverse perinatal outcome by fetal biometry: comparison of customized and population-based standards

OBJECTIVE

To compare the predictive performance of estimated fetal weight (EFW) percentiles, according to eight growth standards, to detect fetuses at risk for adverse perinatal outcome.

METHODS

This was a retrospective cohort study of 3437 African-American women. Population-based (Hadlock, INTERGROWTH-21^st , World Health Organization (WHO), Fetal Medicine Foundation (FMF)), ethnicity-specific (Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD)), customized (Gestation-Related Optimal Weight (GROW)) and African-American customized (Perinatology Research Branch (PRB)/NICHD) growth standards were used to calculate EFW percentiles from the last available scan prior to delivery. Prediction performance indices and relative risk (RR) were calculated for EFW < 10^th and > 90^th percentiles, according to each standard, for individual and composite adverse perinatal outcomes. Sensitivity at a fixed (10%) false-positive rate (FPR) and partial (FPR < 10%) and full areas under the receiver-operating-characteristics curves (AUC) were compared between the standards.

RESULTS

Ten percent (341/3437) of neonates were classified as small-for-gestational age (SGA) at birth, and of these 16.4% (56/341) had at least one adverse perinatal outcome. SGA neonates had a 1.5-fold increased risk of any adverse perinatal outcome (P < 0.05). The screen-positive rate of EFW < 10^th percentile varied from 6.8% (NICHD) to 24.4% (FMF). EFW < 10^th percentile, according to all standards, was associated with an increased risk for each of the adverse perinatal outcomes considered (P < 0.05 for all). The highest RRs associated with EFW < 10^th percentile for each adverse outcome were 5.1 (95% CI, 2.1-12.3) for perinatal mortality (WHO); 5.0 (95% CI, 3.2-7.8) for perinatal hypoglycemia (NICHD); 3.4 (95% CI, 2.4-4.7) for mechanical ventilation (NICHD); 2.9 (95% CI, 1.8-4.6) for 5-min Apgar score < 7 (GROW); 2.7 (95% CI, 2.0-3.6) for neonatal intensive care unit (NICU) admission (NICHD); and 2.5 (95% CI, 1.9-3.1) for composite adverse perinatal outcome (NICHD). Although the RR CIs overlapped among all standards for each individual outcome, the RR of composite adverse perinatal outcome in pregnancies with EFW < 10^th percentile was higher according to the NICHD (2.46; 95% CI, 1.9-3.1) than the FMF (1.47; 95% CI, 1.2-1.8) standard. The sensitivity for composite adverse perinatal outcome varied substantially between standards, ranging from 15% for NICHD to 32% for FMF, due mostly to differences in FPR; this variation subsided when the FPR was set to the same value (10%). Analysis of AUC revealed significantly better performance for the prediction of perinatal mortality by the PRB/NICHD standard (AUC = 0.70) compared with the Hadlock (AUC = 0.66) and FMF (AUC = 0.64) standards. Evaluation of partial AUC (FPR < 10%) demonstrated that the INTERGROWTH-21^st standard performed better than the Hadlock standard for the prediction of NICU admission and mechanical ventilation (P < 0.05 for both). Although fetuses with EFW > 90^th percentile were also at risk for any adverse perinatal outcome according to the INTERGROWTH-21^st (RR = 1.4; 95% CI, 1.0-1.9) and Hadlock (RR = 1.7; 95% CI, 1.1-2.6) standards, many times fewer cases (2-5-fold lower sensitivity) were detected by using EFW > 90^th percentile, rather than EFW < 10^th percentile, in screening by these standards.

CONCLUSIONS

Fetuses with EFW < 10^th percentile or EFW > 90^th percentile were at increased risk of adverse perinatal outcomes according to all or some of the eight growth standards, respectively. The RR of a composite adverse perinatal outcome in pregnancies with EFW < 10^th percentile was higher for the most-stringent (NICHD) compared with the least-stringent (FMF) standard. The results of the complementary analysis of AUC suggest slightly improved detection of adverse perinatal outcome by more recent population-based (INTERGROWTH-21^st ) and customized (PRB/NICHD) standards compared with the Hadlock and FMF standards. Published 2019. This article is a U.S. Government work and is in the public domain in the USA.

The relationship between low birthweight and childhood health: disparities by race, ethnicity, and national origin

PURPOSE

Racial/ethnic disparities in rates of low birthweight (LBW) are well established, as are racial/ethnic differences in health outcomes over the life course. Yet, there is little empirical work examining whether the consequences of LBW for subsequent child health vary by race, ethnicity, and national origin.

METHODS

Using data from the 1998-2016 National Health Interview Survey, we examined whether racial, ethnic, and national differences existed in the association between LBW and subsequent health outcomes, namely being diagnosed with a developmental disability, asthma diagnosis, and poorer general health.

RESULTS

Children born with LBW consistently had poorer health relative to children born with normal birthweight. There was no systematic evidence that the linkages between LBW and subsequent health were weaker for one racial/ethnic/national origin group relative to others.

CONCLUSIONS

LBW was associated with subsequent poorer health. There was no systematic evidence that the link between LBW and subsequent child health were weaker for one racial/ethnic/national origin group relative to others. Together, these findings highlight the importance of reducing race/ethnic disparities in rates of LBW as a way of eradicating inequalities in childhood health.

Customized growth charts: rationale, validation and clinical benefits

Appropriate standards for the assessment of fetal growth and birthweight are central to good clinical care, and have become even more important with increasing evidence that growth-related adverse outcomes are potentially avoidable. Standards need to be evidence based and validated against pregnancy outcome and able to demonstrate utility and effectiveness. A review of proposals by the Intergrowth consortium to adopt their single international standard finds little support for the claim that the cases that it identifies as small are due to malnutrition or stunting, and substantial evidence that there is normal physiologic variation between different countries and ethnic groups. It is possible that the one-size-fits-all standard ends up fitting no one and could be harmful if implemented. An alternative is the concept of country-specific charts that can improve the association between abnormal growth and adverse outcome. However, such standards ignore individual physiologic variation that affects fetal growth, which exists in any heterogeneous population and exceeds intercountry differences. It is therefore more logical to adjust for the characteristics of each mother, taking her ethnic origin and her height, weight, and parity into account, and to set a growth and birthweight standard for each pregnancy against which actual growth can be assessed. A customized standard better reflects adverse pregnancy outcome at both ends of the fetal size spectrum and has increased clinicians' confidence in growth assessment, while providing reassurance when abnormal size merely represents physiologic variation. Rollout in the United Kingdom has proceeded as part of the comprehensive Growth Assessment Protocol (GAP), and has resulted in a steady increase in antenatal detection of babies who are at risk because of fetal growth restriction. This in turn has been accompanied by a year-on-year drop in stillbirth rates to their lowest ever levels in England. A global version of customized growth charts with over 100 ethnic origin categories is being launched in 2018, and will provide an individualized, yet universally applicable, standard for fetal growth.

Customized Fetal Growth Charts for Parents' Characteristics, Race, and Parity by Quantile Regression Analysis: A Cross-sectional Multicenter Italian Study

OBJECTIVES

The purpose of this study was to construct fetal biometric charts between 16 and 40 weeks' gestation that were customized for parental characteristics, race, and parity, using quantile regression analysis.

METHODS

In a multicenter cross-sectional study, 8070 sonographic examinations from low-risk pregnancies between 16 and 40 weeks' gestation were analyzed. The fetal measurements obtained were biparietal diameter, head circumference, abdominal circumference, and femur diaphysis length. Quantile regression was used to examine the impact of parental height and weight, parity, and race across biometric percentiles for the fetal measurements considered.

RESULTS

Paternal and maternal height were significant covariates for all of the measurements considered (P < .05). Maternal weight significantly influenced head circumference, abdominal circumference, and femur diaphysis length. Parity was significantly associated with biparietal diameter and head circumference. Central African race was associated with head circumference and femur diaphysis length, whereas North African race was only associated with femur diaphysis length.

CONCLUSIONS

In this study we constructed customized biometric growth charts using quantile regression in a large cohort of low-risk pregnancies. These charts offer the advantage of defining individualized normal ranges of fetal biometric parameters at each specific percentile corrected for parental height and weight, parity, and race. This study supports the importance of including these variables in routine sonographic screening for fetal growth abnormalities.

Fetal growth: a review of terms, concepts and issues relevant to obstetrics

The perinatal literature includes several potentially confusing and controversial terms and concepts related to fetal size and growth. This article discusses fetal growth from an obstetric perspective and addresses various issues including the physiologic mechanisms that determine fetal growth trajectories, known risk factors for abnormal fetal growth, diagnostic and prognostic issues related to restricted and excessive growth and temporal trends in fetal growth. Also addressed are distinctions between fetal growth 'standards' and fetal growth 'references', and between fetal growth charts based on estimated fetal weight vs those based on birth weight. Other concepts discussed include the incidence of fetal growth restriction in pregnancy (does the frequency of fetal growth restriction increase or decrease with increasing gestation?), the obstetric implications of studies showing associations between fetal growth and adult chronic illnesses (such as coronary heart disease) and the need for customizing fetal growth standards.

A global reference for fetal-weight and birthweight percentiles

BACKGROUND

Definition of small for gestational age in various populations worldwide remains a challenge. References based on birthweight are deficient for preterm births, those derived from ultrasound estimates might not be applicable to all populations, and the individualised reference can be too complex to use in developing countries. Our aim was to create a generic reference for fetal weight and birthweight that overcame these deficiencies and could be readily adapted to local populations.

METHODS

We used the fetal-weight reference developed by Hadlock and colleagues and the notion of proportionality proposed by Gardosi and colleagues and made the weight reference easily adjustable according to the mean birthweight at 40 weeks of gestation for any local population. For application and validation, we used data from 24 countries in Africa, Latin America, and Asia that participated in the 2004-08 WHO Global Survey on Maternal and Perinatal Health (237,025 births). We compared our reference with that of Hadlock and colleagues (non-customised) and with that of Gardosi and colleagues (individualised). For every reference, the odds ratio (OR) of adverse perinatal outcomes (stillbirths, neonatal deaths, referral to higher-level or special care unit, or Apgar score lower than 7 at 5 min) for infants who were small for gestational age versus those who were not was estimated with multilevel logistic regression.

FINDINGS

OR of adverse outcomes for infants small for gestational age versus those not small for gestational age was 1·59 (95% CI 1·53-1·66) for the non-customised fetal-weight reference compared with 2·87 (2·73-3·01) for our country-specific reference, and 2·84 (2·71-2·99) for the fully individualised reference.

INTERPRETATION

Our generic reference for fetal-weight and birthweight percentiles can be easily adapted to local populations. It has a better ability to predict adverse perinatal outcomes than has the non-customised fetal-weight reference, and is simpler to use than the individualised reference without loss of predictive ability.

FUNDING

None.

Obstetrical and neonatal outcomes of pregnancies following gastric bypass surgery: a retrospective cohort study in a French referral centre

BACKGROUND

The objective of this study was to analyze obstetrical and neonatal outcomes following Roux en Y Gastric Bypass procedures (RYGBP).

METHODS

A retrospective cohort study was conducted in a single French tertiary perinatal care and bariatric center. The study involved 24 pregnancies, following RYGBP (exposed group) and two different control groups (non-exposed groups). A body mass index (BMI)-matched control group included 120 pregnancies matched for age, parity, and pregnancy BMI. A normal BMI control group included 120 pregnancies with normal BMI (18.5-24.9 kg/m(2)), matched for age and parity. Hospital data were reviewed from all groups in the same 6-year period. Obstetrical and neonatal outcomes after RYGB were compared, separately, to the two different-matched control groups.

RESULTS

The median interval from RYGBP to conception was 26.6 (range: 3-74) months. Rates of perinatal complications did not differ significantly between the RYGBP group and normal BMI and BMI-matched controls groups. The rate of Cesarean section before labor was higher in the RYGBP patients than in the normal BMI control group (25% vs. 9.3% respectively, p = 0.04). Weight gain was lower in the RYGBP patients than normal BMI control group (5.8 kg vs. 13.2 kg respectively, p < 0.0001). Birthweight was also lower in the RYGBP group than those in normal BMI and BMI-matched controls groups (2,948.2 g vs. 3,368.2 g and 3,441.8 g, respectively, p < 0.0001).

CONCLUSIONS

RYGBP surgery was associated with reduced birthweight, suggesting a possible role of nutritional growth restriction in pregnancy.

Assessing the value of customized birth weight percentiles

Customized birth weight percentiles are weight-for-gestational-age percentiles that account for the influence of maternal characteristics on fetal growth. Although intuitively appealing, the incremental value they provide in the identification of intrauterine growth restriction (IUGR) over conventional birth weight percentiles is controversial. The objective of this study was to assess the value of customized birth weight percentiles in a simulated cohort of 100,000 infants aged 37 weeks whose IUGR status was known. A cohort of infants with a range of healthy birth weights was first simulated on the basis of the distributions of maternal/fetal characteristics observed in births at the Royal Victoria Hospital in Montreal, Canada, between 2000 and 2006. The occurrence of IUGR was re-created by reducing the observed birth weights of a small percentage of these infants. The value of customized percentiles was assessed by calculating true and false positive rates. Customizing birth weight percentiles for maternal characteristics added very little information to the identification of IUGR beyond that obtained from conventional weight-for-gestational-age percentiles (true positive rates of 61.8% and 61.1%, respectively, and false positive rates of 7.9% and 8.5%, respectively). For the process of customization to be worthwhile, maternal characteristics in the customization model were shown through simulation to require an unrealistically strong association with birth weight.

The customised growth potential: an international research tool to study the epidemiology of fetal growth

Customised centiles based on individual fetal growth potential enhance our ability to differentiate between physiological and pathological smallness. A series of studies in different maternity populations has found striking similarities in the way fetal growth varies with maternal and pregnancy related characteristics, and has established the clear advantages of this method over generic, population-based birthweight or fetal weight standards. The method opens up many new avenues for the retrospective study of risk factors and fetal growth. The findings quantify the strength of association between fetal growth restriction and perinatal outcome, and therefore also highlight the clinical imperative to improve antenatal detection of the at-risk fetus. Applied prospectively as customised charts, the concept improves the detection of fetal growth restriction and reduces the need for unnecessary investigations.

Distinguishing pathological from constitutional small for gestational age births in population-based studies

BACKGROUND

Small for gestational age (SGA) can occur following a pathological process or may represent constitutionally small fetuses. However, distinguishing these processes is often difficult, especially in large studies, where the term SGA is often used as a proxy for restricted fetal growth. Since biologic variation in fetal size is largely a third trimester phenomenon, we hypothesized that the definition of SGA at term may include a sizeable proportion of constitutionally small fetuses. In contrast, since biologic variation in fetal size is not fully expressed in (early) preterm gestations, it is plausible that SGA in early preterm gestations would comprise a large proportion of growth restricted fetuses.

AIM

We compared mortality and morbidity rates between SGA and appropriate for gestational age (AGA) babies.

SUBJECTS

A population-based study of over 19million non-malformed, singleton births (1995-04) in the United States was performed. Gestational age (24-44weeks) was based on a clinical estimate. SGA and AGA were defined as sex-specific birthweight <10th and 25-74th centiles, respectively, for gestational age. All analyses were adjusted for a variety of confounding factors.

OUTCOME MEASURES

Excess mortality risk in SGA and AGA babies.

RESULTS

On an additive scale, stillbirth and neonatal mortality rates were higher at every preterm gestation among SGA than AGA births, and similar at term gestations. An inverse relationship between gestational age and excess deaths between SGA and AGA babies delivered at <37weeks was evident.

CONCLUSIONS

In early preterm gestations, the definition of SGA may well be justified as a proxy for IUGR. In contrast, SGA babies that are delivered at term are likely to be constitutionally small.

Adverse pregnancy outcome and association with small for gestational age birthweight by customized and population-based percentiles

OBJECTIVE

The objective of the study was to investigate the association between pregnancy complications and small for gestational age (SGA) birthweight, comparing SGA based on the customized growth potential with SGA based on the birthweight standard of the same population.

STUDY DESIGN

This was a retrospective analysis of a database from a US multicenter study. Pregnancy complications included threatened preterm labor, antepartum hemorrhage, pregnancy-induced hypertension, preeclampsia, stillbirth, and early neonatal death.

RESULTS

Compared with SGA by the birthweight standard, SGA by customized growth potential showed higher risk for each of the 6 adverse indicators. A third of the SGA group was small by customized centiles but not by population-based centiles, yet was strongly associated with each of the pregnancy complications studied. This subgroup of unrecognized SGA babies included 26% preterm deliveries. In contrast, a subgroup that was SGA by the population standard but not by the customized standard (17.2%), was not associated with any of the indicators of adverse outcome.

CONCLUSION

SGA defined by customized growth potential improves the differentiation between physiologically and pathologically small babies.

A customized standard to assess fetal growth in a US population

OBJECTIVE

The objective of the study was to assess the factors that affect fetal growth and birthweight, and to derive coefficients for a customized growth chart applicable in an American population.

STUDY DESIGN

This was a prospective cohort study of 35,235 pregnancies. Coefficients for physiological and pathological variables were derived by backward multiple regression.

RESULTS

The expected birthweight at 40.0 weeks for a standard-size primiparous mother of European origin in an uncomplicated pregnancy was 3453.4 g, very similar to the standardized birthweight observed in other populations. Physiological coefficients were derived for maternal height, weight, parity, ethnic origin, and sex of the baby. Smoking, history of preterm delivery, and hypertensive diseases in the current pregnancy all had negative effects on birthweight, whereas babies of diabetic mothers weighed more. Low as well as high body mass index was associated with birthweight deficit at term.

CONCLUSION

Coefficients that allow determination of the customized growth potential, individually adjusted and excluding known pathological factors, have been derived. Babies of obese mothers have an increased risk of not reaching their fetal growth potential.

An outcome-based approach for the creation of fetal growth standards: do singletons and twins need separate standards?

Contemporary fetal growth standards are created by using theoretical properties (percentiles) of birth weight (for gestational age) distributions. The authors used a clinically relevant, outcome-based methodology to determine if separate fetal growth standards are required for singletons and twins. All singleton and twin livebirths between 36 and 42 weeks’ gestation in the United States (1995–2002) were included, after exclusions for missing information and other factors (n = 17,811,922). A birth weight range was identified, at each gestational age, over which serious neonatal morbidity and neonatal mortality rates were lowest. Among singleton males at 40 weeks, serious neonatal morbidity/mortality rates were lowest between 3,012 g (95% confidence interval (CI): 3,008, 3,018) and 3,978 g (95% CI: 3,976, 3,980). The low end of this optimal birth weight range for females was 37 g (95% CI: 21, 53) less. The low optimal birth weight was 152 g (95% CI: 121, 183) less for twins compared with singletons. No differences were observed in low optimal birth weight by period (1999–2002 vs. 1995–1998), but small differences were observed for maternal education, race, parity, age, and smoking status. Patterns of birth weight-specific serious neonatal morbidity/neonatal mortality support the need for plurality-specific fetal growth standards.

Perinatal outcomes in a South Asian setting with high rates of low birth weight

BACKGROUND

It is unclear whether the high rates of low birth weight in South Asia are due to poor fetal growth or short pregnancy duration. Also, it is not known whether the traditional focus on preventing low birth weight has been successful. We addressed these and related issues by studying births in Kaniyambadi, South India, with births from Nova Scotia, Canada serving as a reference.

METHODS

Population-based data for 1986 to 2005 were obtained from the birth database of the Community Health and Development program in Kaniyambadi and from the Nova Scotia Atlee Perinatal Database. Menstrual dates were used to obtain comparable information on gestational age. Small-for-gestational age (SGA) live births were identified using both a recent Canadian and an older Indian fetal growth standard.

RESULTS

The low birth weight and preterm birth rates were 17.0% versus 5.5% and 12.3% versus 6.9% in Kaniyambadi and Nova Scotia, respectively. SGA rates were 46.9% in Kaniyambadi and 7.5% in Nova Scotia when the Canadian fetal growth standard was used to define SGA and 6.7% in Kaniyambadi and < 1% in Nova Scotia when the Indian standard was used. In Kaniyambadi, low birth weight, preterm birth and perinatal mortality rates did not decrease between 1990 and 2005. SGA rates in Kaniyambadi declined significantly when SGA was based on the Indian standard but not when it was based on the Canadian standard. Maternal mortality rates fell by 85% (95% confidence interval 57% to 95%) in Kaniyambadi between 1986-90 and 2001-05. Perinatal mortality rates were 11.7 and 2.6 per 1,000 total births and cesarean delivery rates were 6.0% and 20.9% among live births >or= 2,500 g in Kaniyambadi and Nova Scotia, respectively.

CONCLUSION

High rates of fetal growth restriction and relatively high rates of preterm birth are responsible for the high rates of low birth weight in South Asia. Increased emphasis is required on health services that address the morbidity and mortality in all birth weight categories.