BOSTON -- It sounds like an old wives' tale, but a boy is significantly more likely to be the second child if a woman gains weight after a first pregnancy, found researchers here.
BOSTON, Sept. 25 -- It sounds like an old wives' tale, but a boy is significantly more likely to be the second child if a woman gains weight after a first pregnancy, found researchers here.
In women whose body mass index increased by more than 3 kg/m2 between a first and second pregnancy, the odds ratio of male conception increased from 1.024 to 1.080, Eduardo Villamor, M.D., of Harvard, and colleagues, reported in the current issue of Fertility and Sterility.
"There could be a causal relation between prepregnant maternal weight gain and the sex ratio of the offspring," concluded the authors, whose findings emerged from the Swedish Birth Registry.
A change in maternal smoking status, a second focus of the investigation, did not influence the sex ratio, the authors added.
Little is known about factors that influence the sex of a fetus, but interest in the issue has increased in reaction to data showing a decline in the male:female sex ratio in several industrialized countries, Dr. Villamor and colleagues stated.
Some evidence has linked parental smoking status changes in sex ratio, but rates of smoking during pregnancy have generally declined in countries with decreasing sex ratios. In contrast, obesity has reached epidemic proportion in some of the nations that have reported declines in male:female sex ratio.
Studies of animal behavior have suggested that maternal nutrition status about the time of conception might influence the offspring's sex. Some epidemiologic data also have indicated that maternal nutrition status might affect sex ratio. However, the authors emphasized that "there is no substantial evidence to believe that the association between women's nutritional status around the time of conception and the gender of the offspring could be causal."
Dr. Villamore and colleagues examined potential influences on sex ratio in data from the Swedish Birth Registry. The study encompassed 220,889 women who had two consecutive singleton births between 1992 and 2004. Live births and stillbirths were included in the analysis.
The birth registry data included the women's weight and height at the first antenatal visit of both pregnancies. Smoking status was ascertained from data in birth and education registries.
The primary finding from the data analysis was that "the proportion of male babies born to the second pregnancy increased linearly with the amount of weight that mothers gained from the first pregnancy," from 1.024 in women who lost more than 1 unit BMI to 1.080 in women who gained 3 or more units (P for the trend, 0.0009).
The association was independent of parental age, length of interpregnancy interval, gender or survival status of the first born, maternal smoking, and major pregnancy and perinatal complications.
In contrast, changes in sex ratio had no association with maternal smoking status during the first and second pregnancy.
The authors reviewed multiple hypotheses for the association between maternal BMI and sex ratio, but they concluded that any explanations for the association are purely speculative.