Introduction
Preterm birth (PTB) and low birthweight (LBW) are related to perinatal mortality, and have been associated with a wide range of adverse developmental outcomes for children.1 Recently, much focus has been on the potential impact of maternal distress and anxiety on birth outcomes. Studies have reported that the prevalence of maternal anxiety during pregnancy varies from 6.6 to 10.4%.2,3Further, maternal anxiety during pregnancy is associated with birthweight and gestational age.4,5 Prenatal anxiety is recognized as a potential risk factor for PTB and LBW.6,7,8
While associations between prenatal maternal anxiety and PTB and LBW have been reported, causal inferences are only tentative. For several reasons, randomization of anxiety or distress during pregnancy cannot be conducted.9 Thus, in humans, investigations are limited to observational designs. Using this methodology, associations between prenatal maternal anxiety and PTB and LBW have been established. New approaches to estimate the effects of prenatal maternal anxiety on childhood outcomes are crucial in advancing the field.9,10
In this study, we estimate the effects of degree (no symptoms, light symptoms and severe symptoms) and timing of prenatal maternal anxiety on infant birthweight and preterm birth. Timing was accounted for by examining whether symptoms were reported at either or both of the 17th or 30th gestational weeks. We test these associations using a sibling comparison design. The design, which involves studying birth outcomes following differential exposure to anxiety during pregnancy across siblings, has been found to reduce the extent of the impact of family-level confounding factors.9-11 on examinations of prenatal anxiety effects.