Within-mother analysis of seasonal patterns in health at birth
Edited by Kenneth W. Wachter, University of California, Berkeley, CA, and approved June 4, 2013 (received for review April 22, 2013)
Abstract
A large literature describes relationships between month of birth, birth weight, and gestation. These relationships are hypothesized to reflect the causal impact of seasonal environmental factors. However, recent work casts doubt on this interpretation by showing that mothers with lower socioeconomic status are more likely to give birth in months that are associated with poorer birth outcomes. Seasonality in the numbers of conceptions in different months can also induce a mechanical correlation between preterm birth and month of birth. This paper analyzes the seasonality of health at birth using a large sample of 647,050 groups of US siblings representing 1,435,213 children. By following the same mother over time, we eliminate differences in fixed maternal characteristics as an explanation for seasonal differences in health at birth. We find a sharp trough in gestation length among babies conceived in May, which corresponds to an increase in prematurity of more than 10%. Birth weight conditional on gestation length, however, is found to be strongly hump-shaped over the year, with 8–9 additional g for summer conceptions. We examine several potential mechanisms for explaining seasonality in birth outcomes that have generally been dismissed in the literature on seasonality in rich countries, notably disease prevalence and nutrition. The May trough in gestation length coincides with a higher influenza prevalence in January and February, when these babies are nearing full term, whereas the hump shape in birth weight is associated with a similar pattern in pregnancy weight gain.
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Published online: July 8, 2013
Published in issue: July 23, 2013
Notes
This article is a PNAS Direct Submission.
*The sole previous paper using sibling comparisons that we are aware of is by Gavrilov and Gavrilova (14), who collected a sample of centenarians, their siblings, and spouses through internet search and found that centenarians are more likely than shorter-lived siblings to have been born in the fall. There may be some measurement issues involved with reconstructing siblingship groups in this fashion. More importantly, their analysis is at the birth month level and therefore, prone to the potentially strong conception rate confounder as discussed below.
†
There is still much uncertainty concerning the association between temperature and birth outcomes. The work by Strand et al. (6) has a review.
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The authors declare no conflict of interest.
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