Women who gained little weight during pregnancy were found to produce a much lower proportion of male fetuses than women who gained higher amounts of weight.
Study researcher Kristen Navara - of the College of Agricultural and Environmental Sciences at the University of Georgia - explains that males have higher vulnerability to adverse conditions in the womb than females.
"Males have an overall higher risk of miscarriage and preterm birth," she says. "In addition, male and female embryos are different metabolically and grow at different rates."
Navara points out that past research in non-mammals has indicated that restricted calorie intake leads to lower production of males than females. However, she says very few studies have looked at the relationship between calorie intake, weight gain and birth sex ratios in humans.
"Thus," Navara adds, "it remains unclear whether subtle differences in food intake and weight gain during gestation differentially influence male survival in utero and, if so, when during gestation these influences would be most potent."
With this in mind, she decided to investigate how weight gain during pregnancy influences birth sex ratios and the sex ratios of fetal deaths.
Study indicates maternal weight gain and sex ratio at birth are 'directly related'
Navara analyzed data from the Centers for Disease Control and Prevention (CDC) Vital Statistics website, which provided information on more than 68 million singleton births in the US between 1990 and 2012.
From this data, she was able to determine the amount of weight each mother gained during pregnancy, the race of the mother, the sex of the infant and the gestational age of the infant at birth.
Results of the analysis, published in the journal PLOS ONE, revealed that women who gained less than 20 pounds during pregnancy produced a much lower proportion of male offspring than female offspring, as well as a much lower proportion of male offspring than women who gained more than 20 pounds.
Commenting on this finding, Navara says:
"The correlation was a near perfect relationship where the proportion of males rose with the number of pounds women gained during gestation. To me, that tight of a relationship indicates that weight gain and the sex ratio at birth are, in fact, directly related and that the relationship isn't driven by another related variable."
Speaking of the potential reasons behind this finding, Navara says it may because male embryos and fetuses have higher metabolic rates, meaning they need higher levels of caloric energy for successful development.
The study also revealed that at 6 months' gestation, the rate of male fetal deaths was much higher among women who gained little weight during pregnancy than those who gained high amounts of weight.
Although data on fetal deaths prior to 6 months' gestation was unavailable, Navara believes that maternal weight gain may be a bigger influence on male fetal death prior to 6 months gestation; no relationship between maternal weight gain and the proportion of male fetal deaths was found at 7 and 8 months' gestation.
Should expectant mothers carrying boys be eating more than those carrying girls?
Guidelines on weight gain during pregnancy from The American College of Obstetricians and Gynecologists do not vary dependent on the sex of a woman's baby.
But Navara says it is well established that growth rates and metabolic rates vary between male and female fetuses, and this latest study indicates that women carrying male fetuses may need a higher caloric intake than those carrying female fetuses.
"I think it is important to continue the research to determine whether women carrying boys should actually be eating more than women carrying girls in order to maximize the chances of the fetus's survival," she adds.
Navara now plans to follow on from this research by monitoring pregnant women and recording their diets and weight gains in order to determine whether specific dietary factors influence fetal survival.
"This would also allow us to determine whether males are even more susceptible at earlier developmental stages than we were able to analyze in the current study," she says. "This would provide us with more information that would tell us 'if you don't get enough of X, this puts male babies at risk of stillbirth at stage Y.'"
In contrast to Navara's findings, Medical News Today recently reported on a study associating maternal obesity with increased risk of infant death.
The researchers of that study, who analyzed data from more than 1.8 million births in Sweden between 1992 and 2010, found there were 5.8 infant deaths per 1,000 births among severely obese mothers and 2.4 infant deaths per 1,000 births among those of a normal weight.