Whether a woman is living with her partner or not when she conceives seems to influence the sex of her child, new research reports.

After reviewing information on more than 86,000 newborns, Dr. Karen Norberg discovered that couples who were not living together or married before their baby was conceived or born were slightly less likely to have a boy.

Norberg cautioned that the effect of parents’ relationship status is “very small,” and couples should not use this information to try to control the gender of their unborn children.

She added that she expects “skepticism and surprise” in reaction to her findings. “But I hope people will be persuaded by the consistency of the results,” she noted.

Although the biological reasons for the connection remain unclear, Norberg explained that babies’ sex may be influenced by relationship factors present during either conception or the pregnancy itself.

For instance, she said that the amount of time parents spend together could affect their hormones, which, in turn, could affect whether the father contributes sperm containing an X or Y, thereby deciding the baby’s gender.

Alternatively, couples that conceive early in the “fertile window” - 4 or 5 days before ovulation - are more likely to have a boy, Norberg said, and couples that live together may tend to have more intercourse during that period.

Finally, male fetuses are more likely to miscarry, at least later in pregnancy, and pregnant, single women may experience more stress, thereby increasing the risk of miscarriage and shifting the sex ratio in favor of girls, Norberg said.

To investigate whether partnership status affects sex ratios, Norberg, who is based at the National Bureau of Economic Research in Cambridge, Massachusetts, studied a sample of 86,436 babies born in the U.S., pooled from five population surveys.

She found that 51.5 percent of couples who were living together or married before the baby was conceived or born gave birth to boys, compared with 49.9 percent of other couples. The results are published in a journal of the Royal Society, London.

She Norberg commented that, over the past 30 years, there has been a slight decline in the percentage of boys born in certain developed countries, including the U.S., Canada, Denmark and the Netherlands. For instance, Norberg said, in the U.S., the sex ratio among white births has changed from 105.9 males per 100 females in 1970, to 105 males per 100 females in 2000.

“It’s possible that the ‘partnership status’ effect could explain the decline, if there has been an increase in births to parents who are not living together,” she pointed out.

SOURCE: Proceedings of the Royal Society of London; Series B: Biological Sciences, 2004.