Older First-Time Mothers More Likely to Bear Low-Weight Babies, UB Study Finds

SNOWBIRD, UTAH -- Women over 35 who give birth for the first time are 50 percent more likely than younger mothers to deliver a pre-term or low-birth-weight baby, one of the first large studies of first-time mothers, conducted by epidemiologists at the University at Buffalo, has found.

The findings indicate clearly that women who become pregnant for the first time past the age of 35 should be considered high-risk and should be monitored closely, said Danelle T. Lobdell, a doctoral candidate in the UB Department of Social and Preventive Medicine and lead author.

The study also showed that, for reasons still unexplained, older first-time mothers are more likely to bear female babies than younger women, who bear more boys than girls.

Germaine Buck, Ph.D., a UB epidemiologist specializing in female infertility and Lobdell's advisor on the research, said 108 boys are born to teen-age mothers for every 100 girls. But the study found that for mothers 35 years old and older, the ratio becomes almost equal.

Results of the study were presented here today at the annual meeting of the Society for Pediatric Epidemiologic Research. They also will be presented here tomorrow at the annual meeting of the Society for Epidemiologic Research.

First births to mothers over 35 increased 124 percent between 1970 and 1986, yet little is known about the outcomes of these pregnancies, Buck said.

"Most statistics on maternal age and health are from teen-agers. Older women have been mostly ignored. And most of the information we have on older mothers comes from women who have had more than one child. These women are said to be at increased risk of adverse pregnancy outcomes. We felt the health statistics may be different for older women giving birth for the first time."

To investigate this hypothesis, Lobdell, Buck and colleagues analyzed the live-birth records of the 43,224 women in New York State, excluding New York City, who were first-time mothers in 1988. Four percent, or 1,692 of the women, were 35 years of age or older.

They assessed the incidence of pre-term delivery and low-birth-weight babies in two age groups: 20-34 years -- considered the prime child-bearing time -- and 35-39 years.

Contrary to their hypothesis, the researchers found that first-time mothers in the older age group were 51 percent more likely to have a pre-term delivery and 54 percent more likely to bear a low-birth-weight baby than the younger group. These results held true after adjusting for known risk factors for these conditions, including multiple births, race, smoking and male gender of infant, Lobdell said.

The researchers are now looking at older first-time mothers' health status during pregnancy to determine if maternal illness, such as hypertension or diabetes, more common in older women, is related to the adverse pregnancy outcomes. They also are looking exclusively at singleton births, which would eliminate the smaller, earlier babies characteristic of multiple births. Twins or triplets are more common among older mothers, who are more likely to need fertility treatments, known to increase the chances of multiple births.

The finding that older mothers are less likely to have boys raises interesting questions, Buck said. "We would like to know where all the boys go."

Arthur M. Michalek, Ph.D., UB associate research professor of social and preventive medicine, also participated in the study.