"The results are provocative because few biological factors are known in humans to influence the chances of either conceiving or carrying to term a baby boy or girl. Our study suggests that maternal nutritional factors might play a role," said Eduardo Villamor, assistant professor of international nutrition at HSPH and lead author of the study.

Some prior studies had looked at what factors might influence the sex ratio, but evidence of causality has been weak. Parental smoking, for example, has been associated with both lower and higher sex ratios. Maternal nutritional status had been studied, but there was little evidence to support a causal relationship with the sex ratio. One of the hypotheses that the authors of this study wanted to test was whether the increase in maternal obesity in several industrialized countries could play a role in the declining sex ratio. Their study found the opposite--maternal weight gain seemed to favor the birth of boys.

The study population, drawn from the Swedish Birth Registry, included 220,889 women who had successive pregnancies between 1992 and 2004 (live births and stillbirths were included). The researchers analyzed the change in women's body mass index (BMI) between the first and second pregnancies. (BMI is weight in kilograms divided by the square of height in meters.) The male to female sex ratio of the second pregnancy increased linearly with the amount of weight change from the first to second pregnancy, from 1.024 in women who lost more than 1 unit BMI to 1.080 in women who gained 3 or more units (a male to female sex ratio of 1.000 would indicate an equal number of boys and girls being born). The trend was independent of obstetric complications, maternal smoking, parental age, length of the interpregnancy interval and the sex or survival status of the first-born child.

The data suggest that interpregnancy weight gain appears related to a slight increase in the probability of giving birth to a baby boy during a second pregnancy. The obesity epidemic does not appear to explain the observed decline in the sex ratio in some industrialized countries, which indicates that there are factors still unknown influencing the probability of giving birth to boys or girls.

The authors are careful to note that women should not gain weight to try to influence the sex of their baby. "Weight gain before pregnancy carries significant risks to the mother and the baby, and should not be practiced to influence the odds of having a boy," said Villamor. "Other factors of which weight gain is only an indicator could be at play here."

Sven Cnattingius, professor of reproductive epidemiology at the Karolinska Institutet in Stockholm, Sweden, was senior author of the study.

The study was supported by grants from the Karolinska Institutet. Villamor is supported in part by the National Institutes of Health and the Department of Nutrition at HSPH.

Link to abstract: http://www.fertstert.org/article/PIIS0015028207030166/abstract.

"Interpregnancy Weight Gain and the Male-to-Female Sex Ratio of the Second Pregnancy: A Population-Based Cohort Study," Eduardo Villamor, Pär Sparén, Sven Cnattingius, Fertility & Sterility, online September 24, 2007.