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Several studies have researched the effects of the environment on prenatal growth and development. One study published in Fertility and Sterility found that women who have their third trimester during the wintertime increase the likelihood of birthing an infant with Vitamin D deficiency tied with poorer bone growth. In a new study, researchers from Princeton University in the United States report that babies who are conceived in the month of May have an increased chance of being born premature.

The study, with authors Janet Currie and Hannes Schwandt from the Center for Health and Wellbeing at the Ivy League University, reviewed 1.4 million siblings that were from 647,050 different mothers who resided in New York City, New Jersey and Pennsylvania. The difference between this study and previous ones is that the authors focused on siblings. By comparing siblings, the researchers are able to better control for genetic and socioeconomic environment factors that could otherwise influence children's growth and health.

The researchers found that for babies conceived in May, there was a 10 percent higher rate of prematurity. Although the researchers were not sure what caused the increased rate, they reasoned that women who get pregnant in May have their third trimester during the wintertime when the flu season is either starting or has already hit. The flu has been tied to causing early births. Even though the researchers could not pinpoint the exact reason, prematurity can be detrimental for a child's growth. Prematurity has been associated to a higher risk of asthma, learning disorders, and several other developmental issues.

Aside from this discovery, the researchers also found that children that were conceived during the summertime tended to weigh more. These babies were around one third of an ounce, or eight grams, heavier, which does not seem like a lot. However, for infants who weigh so little, an extra eight grams have the potential of influencing an infant's health.

"Women gain about one lb. more when they conceive in June, July or August than when they conceive in January," the study wrote according to TIME. "[This suggests] that gains in birth weight are driven, in part, by higher maternal weight gain during pregnancy."

The study was published in the Proceedings of the National Academy of Sciences.