Older Parents Less Likely to Have Male Babies, Study Suggests

March 3, 2000 (Eugene, Ore.) -- Couples who wait to have children may find their chances of having a son decline slightly, according to a study in the March issue of Fertility and Sterility.

Researchers found increasing parental age (particularly the age of the father) is associated with a decrease in the number of births of boys compared to births of girls. The effect of parental age is even stronger in non-whites than in whites.

During the early 1940s, 51.4% of births in the United States resulted in a male child, says Mark Nicolich, PhD, the study's principal author. During the early 1960s this declined to 51.3% of births, while during the early 1990s 51.22% of births resulted in a male infant. Nicolich is a statistician in the epidemiology section of the occupational health division of ExxonMobil Biomedical Sciences Inc., in Annandale, N.J., a subsidiary of Exxon Mobil Corp.

This change in the percentage of male births is relatively small, and it should not affect personal decisions on when to have children, Nicolich says. "People should have children whenever they feel ready for it. While parental age does have an effect on gender, it is a relatively small effect for an individual."

However, this long-term trend showing a declining percentage of male births does raise concern. "There have been several papers hypothesizing about possible causes of declining male birth ratios," Nicolich tells WebMD. "In our research we looked at actual U.S. data and were able to show that a good portion of the decline is associated with older parents and also a change in birth weight."

In this study researchers looked at available data from the National Center for Health Statistics on live births in the United States from 1964 to 1988. They examined data on birth gender, birth weight, parents' age, and race.

Among whites the father's age is twice as important as the mother's age, while among non-whites the influence of father's and mother's age is about equal. There was a relationship between low birth weight and the percentage of non-white male infants, but researchers found no relationship in white infants.

Nicolich urges other researchers to look at parental age and low birth weight as potential reasons for the declining male birth ratio, before speculating about other possible causes.

"Some researchers are hypothesizing that causes such as chemicals in the atmosphere may be a reason for declining male births, without being able to measure the effects. In this paper we're saying, 'Here are some causes we can actually look at and examine.' After looking at these causes, then it will be time to see what those other hypothetical factors may add to the picture -- and researchers should measure them rather than just guessing," says Nicolich.

This study, published in Fertility and Sterility, was purely observational. It did not look at all possible causes of the relationship between parental age, low birth weight, and declining male births.

The research was funded by ExxonMobil Biomedical Sciences Inc.