Many new mothers are often concerned with the month in which their child is born. Births in particular months or seasons have previously been linked to differences in life expectancy, mental health, and educational development to be reached by those children. However, a new study indicates that an infant's month of conception can predict more about his or her health at birth.
A new study compared 1.4 million births of siblings to 647,050 mothers living in New York City, New Jersey, and Pennsylvania — regions with four distinct seasons for comparison. In developed countries like the United States, it is unlikely that nutrition affects an infant's health at birth, as food supplies tend to remain constant. Birth health of siblings born in different months, and therefore conceived in different months, was compared without the variant of mother's health history, as they had the same mother.
The researchers found that infants conceived from January to May have significantly lower birth weights than those conceived in the second half of the year. They found that average birth weight decreases by 10 grams from January to May, but increases by more than 20 grams in June. Infants conceived in the summer season weighed, on average, 8 to 9 grams more than infants born in other seasons. Infants conceived in May have a 10-percent higher rate of being born prematurely than infants conceived in any other month. Related to birth weight, many infants conceived from January to May are born prematurely, while most infants conceived in June and after are born at the appropriate time.
The prevalence of premature births and infants with lower birth weight could be related to seasonal flu. The flu affects most people from November to March or April every year. Often, conception of a child in the early months of the year can ensure a birth during flu season. Influenza infection is known to cause premature births and therefore low birth weight. The viral infection can also compromise a mother's health and cause her to give birth too soon, robbing her child of important development time. This shortening of development creates the outcome of low birth weight, alongside other effects. Previous studies indicate that mothers who contract the flu virus in their first trimester of pregnancy may have children with significant cognitive deficits.
The lack of dangerous diseases in summer months can explain the explosion of birth weight in June, as well as the normalcy of birth weight throughout the rest of the year.
Maternal weight gain was also compared in this study. Regardless of pre-pregnancy weight, mothers who conceived form June to August gained 1 lb. more than those who conceived in other months. And, mothers who conceived in January and December gained the least weight during pregnancy, possibly explaining their children's health outcomes.
While other parts of maternal health were considered, their effect on infant health were not as significant when compared to the effects of month of conception. This is explained by the lack of fluctuation in poor health choices; if a mother smokes or drinks during pregnancy, a change in season will not alter these habits. While this research doesn't condone that expecting mothers start smoking or drinking, it does point out the significance of a mother's overall health based on the changing seasons.
Source: Currie J, Schwandt H. Within-mother analysis of seasonal patterns in health at birth. PNAS. 2013.