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

Associations of maternal ambient temperature exposures during pregnancy with the risk of preterm birth and the effect modification of birth order during the new baby boom: A birth cohort study in Guangzhou, China

Maternal exposures to ambient temperatures during pregnancy may increase the preterm birth (PTB) risk; however, which periods have stronger effects remain controversial. The effects of temperature exposure on PTB in first- and second-born neonate groups may be different during the new baby boom that has followed the Two-child Policy in China. We examined a birth cohort of 4928 pregnant women beginning in 2016 in Guangzhou, China. An inverse distance weighted method was used to estimate the temperature exposure at each individual residential address. A distribution lag non-linear model incorporating a Cox proportional hazard model was employed to estimate the effects of temperature exposure on PTB and test the effects modification of birth order related to the new baby boom. A total of 4101 pregnant women were included, of which 234 (5.7%) experienced PTB. Compared with the mean temperature (23.0 °C), we found a significantly higher risks of PTB associated with high temperatures (i.e, 30 °C [95th centile]) from the 4th to 8th, and 22nd to 27th gestational weeks. A peak effect was found during the 6th week (HR = 1.79, 95% CI: 1.26, 2.54) and 24th week (HR = 1.83, 95% CI: 1.27, 2.62). The risks of PTB were reduced for low temperatures (i.e. 14 °C [5th centile] versus 23.0 °C) from the 2nd to 10th and 20th to 26th gestational weeks, and the negative peak effect was found during the 4th week (HR = 0.43, 95% CI: 0.26, 0.72) and 23rd week (HR = 0.59, 95% CI: 0.43, 0.83). Stratification analyses showed that significant effects of 30 °C versus 23 °C on PTB were observed during the 4th to 8th weeks in the second-born neonate, and the peak effect was found in the 6th week (HR = 2.13, 95% CI: 1.31, 3.47). However, we did not find significant effects of 30 °C during the same weeks in the first-born neonate group. Maternal exposures to higher temperatures during pregnancy may increase the risk of PTB, and lower temperatures may decrease the risk of PTB. Stronger effects of temperature exposures during the first trimester on PTB risk were found among the second-born neonates than among the first-born neonates.

The fragile foundations of the extended fetuses-at-risk approach

BACKGROUND

Whether denominators for postnatal outcomes (ascertained after live birth) with a presumed prenatal origin should consist of fetuses or live births remains controversial. Proponents argue that the extended fetuses-at-risk (FAR) approach (a), provides a justification for medically indicated preterm delivery, (b), avoids paradoxical results, and (c), permits quantification of incidence of fetal-infant phenomena, such as "revealed" small for gestational age (SGA)-which, under FAR, rises with advancing gestation.

METHODS

This conceptual paper examines the validity of the above arguments.

RESULTS

As obstetricians induce babies early because of fetal (or maternal) compromise and despite the dangers posed by immaturity, there is no need to modify a paradigm that portrays preterm birth as a powerful risk factor. The FAR approach generally avoids "paradoxical" intersections because FAR rates of postnatal outcomes depend on the birth rate. However, this property, which causes rates of most postnatal outcomes to rise at term, can also lead to risk reversals and other misleading findings. The FAR formulation does not yield the incidence of postnatal conditions but, rather, the incidence of live birth (and survival to diagnosis) of babies with prevalent conditions (and, sometimes, future ones).

CONCLUSIONS

The proposed arguments do not provide adequate support for extending the FAR approach to postnatal outcomes. As only live births can contribute to the numerator of rates, the usefulness and interpretability of FAR measures in this setting are limited.

Detecting the Effects of Early-Life Exposures: Why Fecundity Matters

Prenatal exposures have meaningful effects on health across the lifecourse. Innovations in causal inference have shed new light on these effects. Here, we motivate the importance of innovation in the characterization of fecundity, and prenatal selection in particular. We argue that such innovation is crucial for expanding knowledge of the fetal origins of later life health. Pregnancy loss is common, responsive to environmental factors, and closely related to maternal and fetal health outcomes. As a result, selection into live birth is driven by many of the same exposures that shape the health trajectories of survivors. Lifecourse effects that are inferred without accounting for these dynamics may be significantly distorted by survival bias. We use a set of Monte Carlo simulations with realistic parameters to examine the implications of prenatal survival bias. We find that even in conservatively specified scenarios, true fetal origin effects can be underestimated by 50% or more. In contrast, effects of exposures that reduce the probability of prenatal survival but improve the health of survivors will be overestimated. The absolute magnitude of survival bias can even exceed small effect sizes, resulting in inferences that beneficial exposures are harmful or vice-versa. We also find reason for concern that moderately sized true effects, underestimated due to failure to account for selective survival, are missing from scientific knowledge because they do not clear statistical significance filters. This bias has potential real-world costs; policy decisions about interventions to improve maternal and infant health will be affected by underestimated program impact.