Weight gain between first and second pregnancies and sex ratio

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.

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.