A mass shooting at Port Arthur, Tasmania, Australia: a study of its impact on early pregnancy losses using a conception time-based methodology

Association between severe cyclone events and birth outcomes in Queensland, Australia, 2008-2018: a population based retrospective cohort study

OBJECTIVE

Investigate an association between severe tropical cyclones (TCs) and birth outcomes in an Australian population.

METHODS

We analysed over 600,000 singleton livebirths collected through the Queensland Perinatal Data Collection between 2008 and 2018. We estimated the odds ratios (ORs) of adverse birth outcomes using logistic multi-level modelling.

RESULTS

Exposure to TCs in early pregnancy was associated with significantly higher odds of preterm births in affected compared to unaffected areas during the TC year [OR=1.28, 95%CI=1.11, 1.49, p=0.001] and slightly significant higher odds in affected areas during TC years compared to non-TC years. Significantly higher odds of low birthweight births were associated with mid-pregnancy exposure to cyclone Marcia [OR=1.62, 95%CI=1.00, 2.40, p=0.016] .

CONCLUSIONS

Findings aligned with studies demonstrating an association between exposure to environmental stressors in early to mid-pregnancy and adverse birth outcomes.

IMPLICATIONS FOR PUBLIC HEALTH

There is limited research into TCs and perinatal health in Australia despite most of the population residing along coastlines and TCs presenting one of the nation's most devastating weather events. This study will inform public health practice and contribute to further research into mitigating environmental risks faced by pregnant women.

Bias due to Selection on Live Births in Studies of Environmental Exposures during Pregnancy: A Simulation Study

BACKGROUND

Studies of the effects of prenatal environmental exposures on postnatal outcomes are particularly vulnerable to live birth bias; i.e., the bias that arises from the necessary restriction of the analysis to live births when that is influenced by both the exposure under study A and unmeasured factors U that also affect the outcome.

OBJECTIVES

In the context of a recent publication of nitrogen dioxide (NO2) and autism spectrum disorder (ASD) that found an odds ratio (OR) of 0.77 per 5.85 ppb NO2 during pregnancy, we aimed to examine what parameters would be needed to account for this protective association through live birth bias.

METHODS

We simulated the magnitude of bias under two selection mechanisms and when both mechanisms co-occur, assuming a true null effect. Simulation input parameters were based on characteristics of the original study and a range of plausible values for the prevalence of unmeasured factor U and the ORs for the selection effects (i.e., the effects of NO2 and U on loss and of U on ASD). Each scenario was simulated 1,000 times.

RESULTS

We found that the magnitude of bias was small when NO2 and U independently influenced pregnancy loss (collider-stratification without interaction), was stronger when NO2-induced loss preferentially occurred in U=1 (depletion of susceptibles), and was strongest when both mechanisms worked together. For example, ORs of 3.0 for NO2-loss, U-loss, U-ASD, and U prevalence=0.75 yielded NO2-ASD ORs per 5.85 ppb NO2 of 0.95, 0.89, and 0.75 for the three scenarios, respectively. The bias is amplified with multiple Us, yielding ORs as low as 0.51.

DISCUSSION

Our simulations illustrate that live birth bias may lead to exposure-outcome associations that are biased downward, where the extent of the bias depends on the fetal selection mechanism, the strength of that selection, and the prevalence of U. https://doi.org/10.1289/EHP7961.

Live-Birth Bias and Observed Associations Between Air Pollution and Autism

A recent analysis found that exposure to air pollution during specific weeks of pregnancy was negatively associated with risk of autism spectrum disorder (ASD) when mutually adjusted for postnatal air-pollution exposure. In this commentary, we describe 2 possible selection-bias processes that might lead to such results, both related to live-birth bias (i.e., the inevitable restriction of the analyzed sample to live births). The first mechanism is described using a directed acyclic graph and relates to the chance of live birth being a common consequence of both exposure to air pollution and another risk factor of ASD. The second mechanism involves preferential depletion of fetuses susceptible to ASD in the higher air-pollution exposure group. We further discuss the assumptions underlying these processes and their causal structures, their plausibility, and other studies where similar phenomena might have occurred.