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Babies conceived in May often born early

Most scientists would bristle at the suggestion that birth signs can affect a baby’s life. But mounting evidence indicates that the month of conception, at least, can play a role in infant well being.

A new study, “Within-mother analysis of seasonal patterns in health at birth,” reports that babies conceived in May are more than 10 percent more likely to be born prematurely than babies conceived during other months. They believe this is, in part, because the third trimester of pregnancy coincides with flu season.

The findings, write authors Janet Currie and Hannes Schwandt, “suggest that […] flu shots might be effective in fighting the seasonal deterioration in length of gestation.”

Currie and Schwandt gathered data from the National Center for Health Statistics to compare the health at birth, by month, of nearly 1.5 million children from about 650,000 mothers. They followed the same mothers over time to compensate for previous findings that mothers with lower socioeconomic statuses tend to give birth during winter months — a time associated with poorer health outcomes. (It is not known why.)

The researchers also found a seasonal pattern in birth weight. “Newborns conceived in the first 5 months of the year have significantly lower birth weight than newborns conceived in the second one-half of the year,” they write. Average weight at birth declines during the first first five months of the year, by 10 grams, increasing by more than 20 grams by the summer months and tapering during the fall.

Low birth weight babies are often premature, so the seasonal pattern in gestation length (i.e. more premature babies born in January and February) may drive this trend. Why should birth weights be higher during the summer months? Birth weights are determined, in part, by the amount of weight the mother gains during pregnancy.

A woman’s average weight increases from January to June by about 0.8 lb. It stays at about this level from June to August, before declining down to the level of January again. “Remarkably, this sizable hump shape hardly changes when additional controls are included and when we do within-mother comparisons,” write Currie and Schwandt. The conception months of highest weight gain, the found, correspond to the months with highest birth weight.

Though some recent studies have suggested that season of birth and poor health outcomes is solely due to the curious phenomenon of women of lower socioeconomic status giving birth in the winter months, this study reaches the opposite conclusion.

“The seasonal effects on birth outcomes are so strong that the bias because of selection is relatively small,” write the authors. So, “a high socioeconomic status mother getting pregnant in an unfavorable month will, on average, experience similarly poor birth outcomes as the typical (lower SES) mother conceiving in this unfavorable month.”

How to address this surprising problem? As a start, if you get pregnant in May, don’t neglect your flu shot come wintertime.

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