Melissa Pandika / Los Angeles Times

It’s still an old wives’ tale that a woman can plan her child’s sex by timing the month when she conceives, but a new study has found that babies conceived at certain times of the year may be predisposed to adverse health outcomes, such as premature birth.

Princeton University health economists Janet Currie and Hannes Schwandt observed a shorter gestation time for infants conceived during the first half of the year, with a “sharp trough” in May, possibly reflecting the spike in seasonal flu cases the following January and February, when their mothers were nearing full term. The researchers saw the highest average weight for infants conceived in the summer, which may be due to seasonal patterns in pregnancy weight gain, Currie said.

Pregnancy length and birth weight “are the most commonly examined measures of infant health at birth, and have been associated with child and adult health outcomes,” Currie and Schwandt wrote in a report published Monday in the journal Proceedings of the National Academy of Sciences.

For almost a century, researchers have investigated the relationship between season of birth and such adult factors as body weight, IQ, mental health, earned income and life expectancy. But these studies didn’t control for maternal characteristics, such as socioeconomic background, that might be confused with the effects of seasonality, Currie said. For that reason, they couldn’t definitively conclude that observed differences were caused by birth season alone.

Currie and Schwandt addressed this limitation by comparing siblings conceived by the same mother at different times throughout the year, allowing them to control for differences among mothers, including education level, smoking, race and marital status. Drawing from publicly available federal birth data, they compared siblings born to 647,050 mothers in New York City, New Jersey and Pennsylvania, representing more than 1.4 million births in all.

The authors looked at conception month rather than birth month because the latter may obscure gestation length, an indicator of whether a mother carried her infant to term. For example, although most babies born in July were likely conceived nine months earlier — in November — it’s possible that some were conceived in December or later and born prematurely.

The study reported that average gestation time dropped relatively steadily each month between January and May, when it measured almost a week shorter than that of infants conceived in January. Average gestation lengths returned to January levels in June, where they remained for the rest of the year. Infants conceived in May had about a 13 percent higher rate of prematurity than those conceived in other months of the year.

Since earlier research had linked flu infections to preterm births, Currie and Schwandt merged their birth data with influenza monitoring data collected by the Centers for Disease Control and Prevention from 1997 onward to find out whether the virus may be to blame for the shorter pregnancy times.

Sure enough, Currie and Schwandt saw that mothers who conceived in the first five months of the year, especially May, were more likely to suffer flu symptoms during the month they gave birth, reflecting the start of the flu season in late August. Babies conceived in May were scheduled to be born in mid-February but tended to suffer a shortened gestation, with deliveries happening in late January and early February, at the height of flu season.