Three alternative methods to resolve paradoxical associations of exposures before term

Reevaluating the protective effect of smoking on preeclampsia risk through the lens of bias

Preeclampsia is a hypertensive disorder that is usually diagnosed after 20 weeks' gestation. Despite the deleterious effect of smoking on cardiovascular disease, it has been frequently reported that smoking has a protective effect on preeclampsia risk and biological explanations have been proposed. However, in this manuscript, we present multiple sources of bias that could explain this association. First, key concepts in epidemiology are reviewed: confounder, collider, and mediator. Then, we describe how eligibility criteria, losses of women potentially at risk, misclassification, or performing incorrect adjustments can create bias. We provide examples to show that strategies to control for confounders may fail when they are applied to variables that are not confounders. Finally, we outline potential approaches to manage this controversial effect. We conclude that there is probably no single epidemiological explanation for this counterintuitive association.

Racial disparities in the development of comorbid conditions after preterm birth: A narrative review

Despite recognition and attempts to reduce racial disparities in perinatal outcomes, Black infants are still disproportionately represented among those who are born preterm. Postnatal investigations of racial disparities in comorbidities and outcomes after preterm birth are increasing, although their results and interpretations are conflicting. In the present review, we 1.) identify important methodological limitations of that literature 2.) summarize the conflicting literature investigating racial disparities, specifically Black-white differences, in postnatal comorbidities and outcomes after preterm birth 3.) describe mechanisms by which racism operates to contextualize our understanding to inform future work to actively reduce disparities in preterm birth and subsequently, its complications.

Ramadan exposure during early pregnancy and risk of stillbirth in Arab women living in Canada

BACKGROUND

Data on fasting during Ramadan and the risk of preterm birth and child mortality are conflicting, but the association with stillbirth is unknown.

OBJECTIVE

We studied the relationship between Ramadan and the risk of stillbirth for Arab women in Quebec, Canada.

METHODS

We conducted a retrospective cohort study using birth certificates for Arab women in Quebec, Canada, between 1981 and 2017. The exposure was Ramadan in the first and second trimester (1-27 weeks of gestation), and the outcome was early (<28 weeks) or late (≥28 weeks) stillbirth. We used log-binomial regression models to estimate risk ratios (RR) and 95% confidence intervals (CI) for the association between Ramadan and risk of stillbirth. We adjusted models for maternal characteristics and assessed associations by cause of death.

RESULTS

The study included 78,349 live births and 274 stillbirths. There were 3.5 stillbirths per 1,000 pregnancies for women exposed to Ramadan between weeks 1-27 of gestation (95% CI 3.0, 4.0), and 3.4 per 1,000 for unexposed women (95% CI 2.8, 4.1). Compared with no exposure, Ramadan between weeks 1-27 was not associated with the risk of early (RR 1.32, 95% CI 0.76, 2.28) or late stillbirth (RR 0.93, 95% CI 0.70, 1.23) in adjusted models. RRs for early stillbirth were 1.40 for Ramadan between weeks 15-21 (95% CI 0.70, 2.80) and 1.38 for Ramadan between weeks 22-27 (95% CI 0.67, 2.84). Relative to no exposure, Ramadan between weeks 15-21 was associated with early stillbirth due to congenital anomaly (RR 3.96; 95% CI 1.35, 11.57) in unadjusted models. There was no association with other causes of stillbirth.

CONCLUSIONS

There is no evidence that Ramadan is associated with the risk of early or late stillbirth overall. Further research is needed to confirm an association with stillbirth due to congenital anomalies.

Association of Race/Ethnicity With Very Preterm Neonatal Morbidities

IMPORTANCE

Severe morbidity in very preterm infants is associated with profound clinical implications on development and life-course health. However, studies of racial/ethnic disparities in severe neonatal morbidities are scant and suggest that these disparities are modest or null, which may be an underestimation resulting from the analytic approach used.

OBJECTIVE

To estimate racial/ethnic differences in severe morbidities among very preterm infants.

DESIGN, SETTING, AND PARTICIPANTS

This population-based retrospective cohort study was conducted in New York City, New York, using linked birth certificate, mortality data, and hospital discharge data from January 1, 2010, through December 31, 2014. Infants born before 24 weeks' gestation, with congenital anomalies, and with missing data were excluded. Racial/ethnic disparities in very preterm birth morbidities were estimated through 2 approaches, conventional analysis and fetuses-at-risk analysis. The conventional analysis used log-binomial regression to estimate the relative risk of 4 severe neonatal morbidities for the racial/ethnic groups. For the fetuses-at-risk analysis, Cox proportional hazards regression with death as competing risk was used to estimate subhazard ratios associating race/ethnicity with each outcome. Estimates were adjusted for sociodemographic factors and maternal morbidities. Data were analyzed from September 5, 2017, to May 21, 2018.

MAIN OUTCOMES AND MEASURES

Four morbidity outcomes were defined using International Classification of Diseases, Ninth Revision, diagnosis and procedure codes: necrotizing enterocolitis, intraventricular hemorrhage, bronchopulmonary dysplasia, and retinopathy of prematurity.

RESULTS

In total, 582 297 infants were included in this study. Of these infants, 285 006 were female (48.9%) and 297 291 were male (51.0%). Using the conventional approach in the very preterm birth subcohort, black compared with white infants had an increased risk of only bronchopulmonary dysplasia (adjusted risk ratio [aRR], 1.34; 95% CI, 1.09-1.64) and a borderline increased risk of necrotizing enterocolitis (aRR, 1.39; 95% CI, 1.00-1.93). Hispanic infants had a borderline increased risk of necrotizing enterocolitis (aRR, 1.39; 95% CI, 0.98-1.96), and Asian infants had an increased risk of retinopathy of prematurity (aRR, 1.85; 95% CI, 1.15-2.97). In the fetuses-at-risk analysis, black infants had a 4.40 times higher rate of necrotizing enterocolitis (95% CI, 2.98-6.51), a 2.73 times higher rate of intraventricular hemorrhage (95% CI, 1.63-4.57), a 4.43 times higher rate of bronchopulmonary dysplasia (95% CI, 2.88-6.81), and a 2.98 times higher rate of retinopathy of prematurity (95% CI, 2.01-4.40). Hispanic infants had an approximately 2 times higher rate for all outcomes, and Asian infants had increased risk only for retinopathy of prematurity (adjusted hazard ratio, 2.43; 95% CI, 1.43-4.11).

CONCLUSIONS AND RELEVANCE

In this study, racial/ethnic disparities in neonatal morbidities among very preterm infants appear to be sizable, but may have been underestimated in previous studies, and may have implications for the future. Understanding these racial/ethnic disparities is important, as they may contribute to inequalities in health and development later in the child's life.

The curse of the perinatal epidemiologist: inferring causation amidst selection

Purpose of review

Human reproduction is a common process and one that unfolds over a relatively short time, but pregnancy and birth processes are challenging to study. Selection occurs at every step of this process (e.g., infertility, early pregnancy loss, and stillbirth), adding substantial bias to estimated exposure-outcome associations. Here we focus on selection in perinatal epidemiology, specifically, how it affects research question formulation, feasible study designs, and interpretation of results.

Recent findings

Approaches have recently been proposed to address selection issues in perinatal epidemiology. One such approach is the ongoing pregnancies denominator for gestation-stratified analyses of infant outcomes. Similarly, bias resulting from left truncation has recently been termed "live birth bias," and a proposed solution is to control for common causes of selection variables (e.g., fecundity, fetal loss) and birth outcomes. However, these approaches have theoretical shortcomings, conflicting with the foundational epidemiologic concept of populations at risk for a given outcome.

Summary

We engage with epidemiologic theory and employ thought experiments to demonstrate the problems of using denominators that include units not "at risk" of the outcome. Fundamental (and commonsense) concerns of outcome definition and analysis (e.g., ensuring that all study participants are at risk for the outcome) should take precedence in formulating questions and analysis approach, as should choosing questions that stakeholders care about. Selection and resulting biases in human reproductive processes complicate estimation of unbiased exposure- outcome associations, but we should not focus solely (or even mostly) on minimizing such biases.

Two denominators for one numerator: the example of neonatal mortality

Preterm delivery is one of the strongest predictors of neonatal mortality. A given exposure may increase neonatal mortality directly, or indirectly by increasing the risk of preterm birth. Efforts to assess these direct and indirect effects are complicated by the fact that neonatal mortality arises from two distinct denominators (i.e. two risk sets). One risk set comprises fetuses, susceptible to intrauterine pathologies (such as malformations or infection), which can result in neonatal death. The other risk set comprises live births, who (unlike fetuses) are susceptible to problems of immaturity and complications of delivery. In practice, fetal and neonatal sources of neonatal mortality cannot be separated-not only because of incomplete information, but because risks from both sources can act on the same newborn. We use simulations to assess the repercussions of this structural problem. We first construct a scenario in which fetal and neonatal factors contribute separately to neonatal mortality. We introduce an exposure that increases risk of preterm birth (and thus neonatal mortality) without affecting the two baseline sets of neonatal mortality risk. We then calculate the apparent gestational-age-specific mortality for exposed and unexposed newborns, using as the denominator either fetuses or live births at a given gestational age. If conditioning on gestational age successfully blocked the mediating effect of preterm delivery, then exposure would have no effect on gestational-age-specific risk. Instead, we find apparent exposure effects with either denominator. Except for prediction, neither denominator provides a meaningful way to define gestational-age-specific neonatal mortality.

Association of Maternal Preeclampsia With Infant Risk of Premature Birth and Retinopathy of Prematurity

Importance

Studies report conflicting associations between preeclampsia and retinopathy of prematurity (ROP). This study provides explanations for the discrepancies to clarify the relationship between preeclampsia and ROP.

Objective

To evaluate the association of maternal preeclampsia and risk of ROP among infants in an unrestricted birth cohort and a restricted subcohort of preterm, very low birth weight (P-VLBW) infants.

Design, Setting, and Participants

A retrospective review of 290 992 live births within the Intermountain Healthcare System in Utah from January 1, 2001, through December 31, 2010, was performed. Generalized estimating equations for logistic regressions with covariate adjustment were applied to relate ROP to preeclampsia among the full cohort and in a subcohort of P-VLBW infants born at younger than 31 weeks' gestation and weighing less than 1500 g.

Main Outcomes and Measures

The occurrence of ROP was related to maternal preeclampsia in the full cohort and in a subcohort of P-VLBW infants.

Results

In the full cohort, 51% of the infants were male and the mean (SD) gestational age was 38.38 (1.87) weeks. In the P-VLBW cohort, 55% were male and the mean (SD) gestational age was 26.87 (2.40) weeks. In the full cohort, preeclampsia was associated with an increased risk of all ROP (adjusted odds ratio [aOR], 2.46; 95% CI, 2.17-2.79; P < .001), severe ROP (aOR, 5.21; 95% CI, 3.44-7.91; P < .001), infant death (aOR, 1.66; 95% CI, 1.16-2.38; P = .006), and giving birth to a P-VLBW infant (aOR, 7.74; 95% CI, 6.92-8.67; P < .001). In the P-VLBW subcohort, preeclampsia was inversely associated with the development of all ROP (aOR, 0.79; 95% CI, 0.68-0.92; P = .003), severe ROP (aOR, 0.62; 95% CI, 0.36-1.06; P = .08), and infant death (aOR, 0.19; 95% CI, 0.11-0.32; P < .001).

Conclusions and Relevance

Preeclampsia was associated with an increased risk of developing ROP among an unrestricted cohort but with a reduced risk of ROP among a restricted subcohort of P-VLBW infants. Although the conflicting associations in the full and P-VLBW cohorts may reflect true differences, the association of a reduced risk of ROP among the P-VLBW subcohort also may reflect biases from restricting the cohort to prematurity, because prematurity is an outcome of preeclampsia.

Disparities in infant hospitalizations in Indigenous and non-Indigenous populations in Quebec, Canada

BACKGROUND

Infant mortality is higher in Indigenous than non-Indigenous populations, but comparable data on infant morbidity are lacking in Canada. We evaluated disparities in infant morbidities experienced by Indigenous populations in Canada.

METHODS

We used linked population-based birth and health administrative data from Quebec, Canada, to compare hospitalization rates, an indicator of severe morbidity, in First Nations, Inuit and non-Indigenous singleton infants (< 1 year) born between 1996 and 2010.

RESULTS

Our cohort included 19 770 First Nations, 3930 Inuit and 225 380 non-Indigenous infants. Compared with non-Indigenous infants, all-cause hospitalization rates were higher in First Nations infants (unadjusted risk ratio [RR] 2.05, 95% confidence interval [CI] 1.99-2.11; fully adjusted RR 1.43, 95% CI 1.37-1.50) and in Inuit infants (unadjusted RR 1.96, 95% CI 1.87-2.05; fully adjusted RR 1.37, 95% CI 1.24-1.52). Higher risks of hospitalization (accounting for multiple comparisons) were observed for First Nations infants in 12 of 16 disease categories and for Inuit infants in 7 of 16 disease categories. Maternal characteristics (age, education, marital status, parity, rural residence and Northern residence) partly explained the risk elevations, but maternal chronic illnesses and gestational complications had negligible influence overall. Acute bronchiolitis (risk difference v. non-Indigenous infants, First Nations 37.0 per 1000, Inuit 39.6 per 1000) and pneumonia (risk difference v. non-Indigenous infants, First Nations 41.2 per 1000, Inuit 61.3 per 1000) were the 2 leading causes of excess hospitalizations in Indigenous infants.

INTERPRETATION

First Nations and Inuit infants had substantially elevated burdens of hospitalizations as a result of diseases of multiple systems. The findings identify substantial unmet needs in disease prevention and medical care for Indigenous infants.