Vigorous leisure activity may reduce risk of preterm birth

CHAPEL HILL — Women who engage in vigorous leisure activity during their first and second trimesters of pregnancy may experience a decreased risk of preterm birth, a new University of North Carolina at Chapel Hill School of Public Health study suggests.

A report on the study appears in the November issue of the journal Epidemiology.

Vigorous leisure activity includes exercise programs such as swimming laps, jogging at a moderate to fast pace, aerobics or aerobic dance, other fast dancing and moderate to fast bicycling. Preterm birth is defined as delivery before completion of 37 weeks gestation.

The study recruited 1,699 women at 24 to 29 weeks gestation of a single pregnancy from UNC Hospitals, Wake County Human Services and Wake Area Health Education Center prenatal clinics between 1995 and 1998.

The type and duration of any regular vigorous leisure activity was assessed through a telephone interview covering the three months before pregnancy and during the first and second trimesters of pregnancy.

Researchers found that 22 percent of participants enjoyed vigorous leisure activity before pregnancy, 14 percent during the first trimester and 8 percent during the second trimester. Activity before pregnancy did not affect the likelihood of preterm birth. The risk of preterm birth was somewhat reduced with vigorous leisure activity during the first trimester, however, and more so during the second trimester. Such activity did not have an impact on postterm delivery.

“We found that few pregnant women chose to do vigorous activity regularly, but for those women that did, their risk of preterm birth was not increased and may even have been reduced,” said Dr. Kelly Evenson, research assistant professor of epidemiology at UNC’s School of Public Health and the study’s lead author.

Evenson and colleagues also found that regular physical activity is more strongly linked with a reduced risk for early preterm birth ? before 34 weeks ? compared with later preterm births (during 34 to 36 weeks gestation). No change in risk was seen with a greater duration of activity.

More study is needed, Evenson said.

“Because women in this cohort determined their own level of activity, we can’t rule out that there is some self-selection going on,” she said. “Women who are feeling better during pregnancy may choose to engage in regular physical activities, while those who do not feel well may choose to be less active. So next steps would be to conduct a randomized clinical trial, as well as other observational studies which include a more detailed look at all types of activities.”

Co-authors of the study are Dr. Anna Maria Siega-Riz, assistant professor of maternal and child health and of nutrition, and Dr. David A. Savitz, professor and chairman of the department of epidemiology, both at UNC’s School of Public Health; Dr. Jenn A. Leiferman, assistant professor of human nutrition, foods and exercise at Virginia Polytechnic Institute and State University; and Dr. John M. Thorp Jr., associate professor of obstetrics and gynecology at UNC’s School of Medicine.

“It’s striking that for whatever reason, there is a fairly strong relationship between being inactive later in pregnancy and increased risk of preterm birth,” Savitz said. “We don’t yet know if it’s causal, but at the very least it’s a lead that’s worth trying to understand more fully.

“It’s especially compelling given that our ability to predict who will or won’t have preterm birth is very weak overall,” Savitz added. “We don’t have a lot of well-established, strong risk factors, and those we do have ? a history of preterm birth or multiple gestation ? can be predicted but not modified. So if there is in fact a causal relationship between activity level and the risk of preterm birth, it’s quite exciting, because we can do something about it. It’s a risk factor amenable to intervention.”