Sunshine, Temperature and Racial Disparity in Birth Outcomes -- Starting at Conception

Recent research has highlighted the complicated effects that season, weather and other environmental factors may have on birth weight, including extreme weather events (e.g., hurricanes and extreme heat) and selection bias (that mothers may plan their pregnancies according to season).  While high temperatures and adverse environmental events have been found to reduce birth weight, sunshine has been found to have positive or negative effects depending on the mother’s race. This finding is consistent with the role that skin pigmentation plays in producing Vitamin D via sunshine balanced against the possible harmful effects of sun exposure, such as immune system impairment and reduced folic acid.  Sunshine may therefore help explain the racial disparity in birth outcomes.  If sunshine also affects the likelihood of conception, as suggested by animal studies of the effects of Vitamin D on fertility, then it may also affect the viability and characteristics of the resulting fetus – and thus its gestation and birth weight. To our knowledge, however, no economic study has considered the effects of sunshine or temperature on conception nor has any study attempted to disentangle their effects on gestation from birth weight.  Our research attempts to close this gap by investigating the effects of weather at the beginning of the birth process – conception – and then systematically moving forward to study its effects on gestation and birth weight. 

Examining the effects of weather leads to several complications that are addressed in our empirical analyses.  Weather has strong geographical and seasonal patterns which may exert independent effects on fertility and birth outcomes, and women may plan their pregnancies with regular weather patterns and season in mind.  Sunshine and temperature may also have complicated, nonlinear effects that differ by day of the week; both may be correlated with air pollution.  By combining daily weather from NASA’s Surface Meteorology and Solar Energy: Daily Averaged Data and birth outcomes data from the 1989-2004 Natality Detail files, our empirical analyses control for these confounding factors in a number of ways.  Our models are subjected to falsification tests that use future weather or weather from a randomly-drawn county as placebos.  We address biological, behavioral and environmental differences by stratifying our samples by such characteristics as maternal race, proxies for planned vs. unplanned pregnancies, and air quality in the county of residence.  Our preliminary results suggest that sunshine during the conception period has strong negative effects on fertility in the summer and early fall months; these effects weaken and eventually become positive towards the end of winter and early spring, when Vitamin D stores are likely to be depleted.  Sunshine during conception also appears to decrease the probability of preterm birth, which suggests that it may affect the viability and health of the fetus and thus birth outcomes in complicated ways not captured by existing studies of birth weight.  Given the critical role that skin pigmentation plays in the effects of sun exposure, these findings suggest that sunshine may help explain racial differences throughout the entire birth process.