Getting pregnant in May raises the risk that your baby will be born prematurely. Conceiving in the summertime, on the other hand, boosts the odds of having a heftier infant.
The new findings add to growing evidence that the timing of pregnancy can influence the health of the baby. Other studies have linked birth month to eventual intelligence, mental health, longevity and even wealth.
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A closer look at previous studies, however, has uncovered a potential flaw: Socioeconomic status seems to have a strong influence on the months that mom deliver their babies, and minority women who are poor, young, less educated and single tend to give birth in the months that have been linked with the worst consequences for babies.
For the new study, researchers from Princeton University got around that problem by looking at siblings born to the same mothers. They collected data on 1.4 million births to nearly 650,000 moms in New York City, New Jersey and Pennsylvania. Then, they compared the outcomes of children conceived to identical moms but at different times of year.