A slightly greater number of males than females are born worldwide every year.
In recent decades, although there are still more baby boys born than girls, there has been an apparent decline in the ratio of male to female newborns in several industrialized countries, including Canada, Denmark, England, Germany, Japan and the United States. That has led researchers to ask: Are there any factors that can influence the probability of giving birth to a baby boy or girl? A new study from the Harvard School of Public Health (HSPH) and Karolinska Institutet in Stockholm, Sweden, found that mothers who experienced an increase in weight from the beginning of the first pregnancy to the beginning of the second pregnancy may be slightly more likely to give birth to a baby boy during their second pregnancy. The study appears online September 24, 2007 in the journal Fertility & Sterility.
“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.