Methodological considerations related to the epidemiologic study of birth outcomes: maternal ART use and adverse birth outcomes

Background A major factor contributing to continued high under-five mortality in sub-Saharan Africa (SSA), is maternal HIV infection, which is associated with adverse birth outcomes such as preterm delivery (PTD), small-for-gestational age (SGA) and low birthweight (LBW) infants. Introduction of antiretroviral therapy (ART) during pregnancy has been a successful intervention for promotion of maternal and infant health, however it has also been linked to an increased risk of adverse birth outcomes. Consequently, the association between maternal ART use and these adverse outcomes is an important area of research in SSA which has the majority of pregnant women living with HIVand the highest rates of PTD, SGA and LBW infants. However, the current state of epidemiologic knowledge remains limited because most evidence on this association comes from observational studies, which have previously given inconsistent findings. Accordingly, the overarching aim of this PhD was to reliably quantify the relationship between maternal ART use and adverse birth outcomes, by addressing the role of methodological factors inherent in observational research in this association. Methods This research included pregnant women (aged ≥18 years) seeking antenatal care at a public sector midwife obstetric unit in Gugulethu (GMOU), Cape Town, enrolled into two separate dedicated research cohort studies. Enrolled women were followed-up during pregnancy and postpartum with their infants, with data obtained from study questionnaires, physical examinations and abstraction of clinical and obstetric records. In parallel, routine electronic data, linked across clinics and data sources were obtained for all pregnant women at the GMOU (pregnancy exposure registry (PER)) and across the Western Cape province (PHDC). Findings The incidence of gestational age (GA) based birth outcomes and the association between maternal ART use and these outcomes, was found to be substantially influenced by method of GA assessment used. While GA based on both last menstrual period (LMP) and measurement of symphysis fundal height (SFH) led to under and over estimation (relative to ultrasound), only LMP-based GA gave rise to a biased measure of association. Across data sources used in this research, an overall PTD incidence of 17% was observed which was lower than incidences observed in the pre-universal ART era. In the cohort studies, there was no significantly increased PTD risk in women living with HIV (compared to living without HIV), predominantly on the tenofovir + emtricitabine + efavirenz regimen. However, when assessed in the significantly larger population of pregnant women (PHDC), an increased PTD risk in women living with HIV was observed. There did not appear to be differences in PTD risk by ART status in the cohort studies or PER. However, across the province those initiating ART preconception were at increased PTD risk compared to those initiating during pregnancy. Blood pressure, particularly when assessed longitudinally played an important role in the association between maternal ART use and PTD, high normal and abnormal blood pressure trajectories associated with increased PTD risk. There did not appear to be any effect modification in the trajectory groups by HIV status for PTD. In the cohort study the overall incidence of SGA infants was 9%, with an increased SGA risk observed in women living with HIV compared to living without HIV. While no differences were observed in SGA risk by ART status, the highest risk was observed among women initiating in the second trimester. An overall LBW incidence of 13% was observed in the cohort study, with no differences observed in risk by HIV status or ART status. Blood pressure also played a role in the occurrence of LBW infants, with abnormal trajectories associated with increased LBW infant risk. Additionally, effect modification among women with abnormal trajectory groups, with women living without HIV at increased risk of LBW infants compared to women living HIV was observed. Finally, investigating this association using both cohort studies and population based electronic health care data proved to be valuable. The three data sources gave similar effect estimates, with varying levels of precision, and each with distinct but complementary benefits. The cohort studies and PER included smaller select groups of women and provided detailed investigation of risk factors that could impact the overall association. In contrast the provincial dataset, had limited risk factor data, but provided overall associations with the ability to detect subtle differences. Conclusion Reassuringly, the magnitude of difference in PTD risk by HIV status under policies of universal maternal ART use, appears to be smaller than in the past. Of concern, however, was the finding of increased SGA risk. Taken together, these findings highlight the need to improve mechanistic understanding of ART-mediated adverse birth outcomes, in order to ensure optimal maternal and infant outcomes. The methodological findings underscore the importance of considering the potential for bias related to selection and measurement when designing and/or evaluating findings from studies investigating this association and by extension other medications in pregnancy

Bayesian Imputation of Missing Covariates

Missing values are a pervasive problem in almost all kinds of studies. In large cohort studies, the type of study most often conducted in the field of epidemiology, missing observations in covariates pose the major challenge. Since measurements are taken in an uncontrolled environment, typically many covariates need to be considered as potential confounders to filter out unwanted influences that environmental factors may have on the estimates of interest. Due to the large number of variables measured and the fact that measurement often relies on participants recalling and reporting detailed information, large proportions of missing data are common in these types of studies. In light of the above, the research that forms this thesis focuses on the analysis of incomplete cohort study data where missingness is in the covariates. We describe a fully Bayesian approach to analyse and impute data in this setting and discuss a number of naive and more sophisticated approaches to impute such data using multiple imputation with chained equations (MICE). The fully Bayesian approach is applied to multiple applications from the field of Epidemiology, and is further extended to settings with time-varying covariates, in which additional challenges, such as the functional form of the association between outcome and covariate and potential endogeneity arise. Moreover, the implementation of the fully Bayesian approach in the R package JointAI is described and illustrated by mean

Haloacetic acids in public drinking water and risk of adverse birth outcomes in the Born in Bradford cohort

Disinfection of drinking water is vital to protect the public against disease. However disinfectants such as chlorine react with organic matter in drinking water to produce a wide range of chemical disinfection by-products (DBPs) of potential health concern including haloacetic acids (HAAs). This thesis is an epidemiologic analysis investigating the relationship between prenatal exposure to HAAs in drinking water and adverse birth outcomes in “Born in Bradford”, a large multi-ethnic prospective birth cohort study based in Bradford, England. It focuses on the understudied and as yet unregulated HAAs which are the second most prevalent class of chlorination DBPs in UK drinking waters. To assess exposure, area-level concentrations to three select HAAs (measured in drinking water samples newly collected for this study, modelled in time and space, and weighted to each cohort woman’s specific trimester of pregnancy by postcode of residence) were combined with individual water consumption information collected via questionnaire at recruitment to the cohort. Despite the benefits of state-of-the-art exposure metrics and a large sample size, this study does not find any significant patterns of association between prenatal exposure to HAAs and either birth weight, being born term low birth weight or small-for-gestational age. Water consumption over the course of late pregnancy was further studied in a subset of cohort women. A small but significant increase in water consumption was reported, bearing in mind that both behaviour change over the third trimester of pregnancy and measurement error likely contributed to this effect. This research addresses some of the limitations of previous DBP studies in terms of exposure assessment and birth outcome definitions, and uniquely evaluates the variability of individual water consumption over time. It also identifies areas for future research and examines the importance of HAAs and birth weight-based outcomes in the larger research context.Open Acces

Preconception nutrition interventions and intrauterine growth: Exploring mechanism and identifying high-risk groups

Impaired intrauterine growth, inability of the fetus to achieve the required growth potential, contributes to a higher burden of neonatal morbidity and mortality. Intrauterine growth is an inferred process and small for gestational age is considered a rough estimate of impaired intrauterine growth that requires reliable gestational age data. Due to sparse data and measurement errors in gestational age, researchers rely on markers such as birth weight, birth length, and birth head circumference to infer fetal growth. While the etiology of impaired fetal growth is multifactorial, maternal anemia and undernutrition contribute substantially to impaired fetal growth and are prioritized in 2030 global nutrition goals by the World Health Assembly. Nutrition supplements such as lipid-based nutrient supplements, multiple micronutrients, and iron or folic acid during pregnancy are presumed to improve maternal anemia. But the effect of the supplements during pregnancy on fetal growth appears to be small to negligible, which has shifted the researcher’s focus to the pre-conception period. However, prior reviews on the preconception period have either synthesized the evidence from observational studies, or have explored outcomes such as congenital anomalies (e.g., neural tube defects), neurodevelopment disorders, or only birth weight. Hence the evidence from existing randomized controlled trials (RCTs) evaluating the effect of preconception nutrition supplements on maternal anemia and all markers of fetal growth including birth weight, birth length, and birth head circumference has not been systematically summarized and synthesized. We bridged this knowledge gap in the current dissertation (Aim 1). The Women First (WF) Preconception Nutrition Trial found that lipid-based nutrient supplementation started preconception or during pregnancy conferred greater benefits for birth weight and birth length among mothers who were anemic (Hemoglobin (Hb) < 12 g/dL) than among mothers who were non-anemic (Hb ≥ 12 g/dL) pre-pregnancy. However, by dichotomizing women into anemic and non-anemic women, we may miss high-risk women with specific Hb levels who may obtain greater benefits for intrauterine growth associated with the supplements. Assessing the effect of preconception nutrient supplements across a range of pre-pregnancy Hb levels will help us identify women with specific Hb cut-offs who may have the greatest potential to respond to the supplements (Aim 2). Additionally, there are major gaps in understanding the mechanisms of how nutrient supplements, consumed either before or during pregnancy, improve intrauterine growth. A better understanding of the underlying mechanisms would allow for fine-tuning of nutrition interventions for greater efficacy. Here, we examined whether Hb during pregnancy could be a potential mechanism through which nutrition supplements improve intrauterine growth (Aim 3). For Aim 1, we undertook a systematic review and meta-analysis of the RCTs evaluating the effect of preconception nutrition supplements on maternal hemoglobin and markers of intrauterine growth including birth weight, birth length, birth head circumference, and small for gestational age. Additionally, we examined preterm birth as an important perinatal outcome. We searched electronic databases including PubMed, Web of Science, Embase, CINAHL, and Cochrane Central. We computed pooled mean differences and risk ratios (RR) with 95% confidence intervals (CIs) using random-effect models. We employed I2 and Cochran’s Q test statistics to assess heterogeneity. We used the GRADE (grading of recommendations, assessment, development, and evaluations) tool to assess the quality of evidence. For Aim 2 and Aim 3, we leveraged the existing data from a large multi-country Women First (WF) Preconception Nutrition Trial conducted in Pakistan, India, Guatemala, and the Democratic Republic of Congo. Women in the WF trial were randomized to consume a lipid-based nutrient supplement (LNS) at least three months before and during pregnancy (Arm 1- preconception), only during pregnancy (Arm 2- during), or not at all (Arm 3 - control). The outcome was weight, length, and head circumference within 48 hours of birth expressed as Z-scores. For Aim 2, we analyzed the WF trial data on 2443 women-newborn dyads. For each site, we computed adjusted mean differences in these Z-scores between the randomized arms across six pre-pregnancy Hb categories (8-8.9, 9-9.9, 10-10.9, 11-11.9, 12-12.9, and ≥13g/dL) based on Hb distributions. We pooled site-specific effect measures using meta-analysis. For Aim 3, hemoglobin measured at 12 (n=2075) and 32 weeks of gestation (n=2157) was a mediator. We employed causal mediation analysis under a counterfactual approach to estimate direct and indirect effects. For Aim 1, we identified 20 eligible RCTs (n=27,659 women). Preconception nutrition supplements (iron and folic acid, multiple micronutrients, and a lipid-based nutrient supplement) increased maternal hemoglobin by 0.30g/dL ((0.03, 0.57); I2=79%). However, we did not find a significant effect of the supplements on birth weight (12.47gm ((-33.14, 58.08); I2=58%)), birth length (0.15cm (-0.26, 0.56); I2=68%; n=5), birth head circumference (-0.23cm (-0.88, 0.43); I2=84%), small for gestational age (RR: 0.91 (0.80,1.04); I2=31%), or preterm birth (RR: 0.93 (0.69,1.25); I2=57%). Overall, the quality of evidence was assessed as moderate and very low for maternal hemoglobin and three markers of intrauterine growth including birth weight, birth length, and birth head circumference, respectively. In Aim 2, we found that the effect of LNS on birth weight, length, and head circumference varied by pre-pregnancy Hb categories. Pooled mean differences in the Z-scores for birth length (0.60 (0.03, 1.23)), birth weight (0.50, (0.11, 0.89)), and birth head circumference ((0.26, (0.02, 0.51)) were greatest for Arm 1-preconception vs. Arm 3-control women with Hb 9-9.9g/dL. Women with Hb 10-10.9g/dL also benefited from preconception LNS. However, compared to controls, the effects of preconception LNS on birth weight, birth length, and birth head circumference attenuated for women with Hb 10-10.9g/dL. Compared to Arm 3-control, LNS during pregnancy (Arm 2) improved birth length, birth weight, and birth head circumference for women with Hb 8-10.9g/dL. Preconception LNS (Arm1) vs. LNS during pregnancy (Arm 2) improved the three markers of intrauterine growth only for women with Hb 9-9.9g/dL. Women with nearly normal (11-11.9 g/dL) and normal Hb (≥12g/dL) did not appreciably benefit from LNS, offered before and or during pregnancy. For Aim 3, Hb at 12 or 32 weeks of gestation did not mediate the relationship between the LNS and intrauterine growth. Indirect effects of preconception LNS (Arm 1) vs. Arm 3, mediated by Hb at 12 weeks, were 0.02 (-0.02, 0.01), 0.01 (-0.01, 0.02), and 0.01 (-0.01, 0.02) for length, weight, and head circumference Z-scores, respectively. The corresponding direct effects, not mediated by Hb, were 0.18 (0.09, 0.33), 0.12 (0.03, 0.23), and 0.06 (-0.03, 0.20), respectively. Site-specific and gestational age-adjusted data analyses both at 12 and 32 weeks of gestation confirmed the findings of negligible mediation by Hb during pregnancy. All types of preconception nutrition supplements studied to date appear to reduce maternal anemia. However, it is uncertain whether there are beneficial effects of preconception nutrition supplements on markers of intrauterine growth. Low quality of evidence from the RCTs examining the markers of intrauterine growth warrants future well-designed RCTs to produce solid scientific data, particularly on the benefits of a more comprehensive package of preconception nutrition supplements that include both macro- and micronutrients. The findings from the WF trial suggest that the benefits of preconception LNS on fetal growth are mainly confined to women with pre-pregnancy Hb 9-9.9g/dL. Compared to controls, women with Hb 10-10.9g/dL also benefited from preconception LNS, albeit the magnitude of effect on three markers of intrauterine growth was modest for women with Hb 10-10.9g/dL. However, LNS, started during pregnancy, appeared to improve markers of intrauterine growth for women with a wider range of Hb 8-10.9g/dL. Women with Hb ≥ 11g/dL did not benefit from LNS started either pre-conception or during pregnancy. These findings suggest that prioritizing women with specific pre-pregnancy Hb categories for targeted nutrition interventions may be advisable. This would target limited resources most efficiently in LMICs. Lastly, negligible mediation by Hb during pregnancy suggests that alternative pathways that potentially mediate the relationship between LNS and intrauterine growth need to be investigated