"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," Eduardo Villamor, assistant professor of international nutrition at HSPH and lead author of the study published in the journal Fertility & Sterility.
The researchers revealed that what they actually wanted to test was whether the increase in maternal obesity in several industrialized countries could play a role in the declining sex ratio.
However, the study showed that maternal weight gain seemed to favour the birth of boys, they said.
The researchers analysed data where in the 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). They analysed the change in women's body mass index (BMI) between the first and second pregnancies.
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, and parental age, length of the interpregnancy interval and the sex or survival status of the first-born child.
The data, therefore suggested that interpregnancy weight gain appears related to a slight increase in the probability of giving birth to a baby boy during the second pregnancy.
It also stated 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 researchers recommended 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. Other factors of which weight gain is only an indicator could be at play here,"Villamor said