A recent study published by the Yale Center for Perinatal, Pediatric and Environmental Epidemiology in the current issue of the American Journal of Epidemiology indicates that for future mothers with asthma, the severity of their condition may be affected by the sex of the fetus. Female fetuses are associated with worse maternal asthma than male fetuses.

Helen Kwon ’96 EPH ’04, the principal author of the study, said the main finding was that pregnant women with asthma carrying a male fetus had approximately 10 percent better lung function than those carrying a female fetus.

Though the work does not presently have direct clinical significance, it points to new biological mechanisms causing asthma, and it may suggest different ways to treat the condition.

“This is the most common disease in pregnancy,” said Michael Bracken, a co-author of the study and professor of epidemiology at Yale. “Some of our other work has shown us between 8 and 9 percent of women who are pregnant are asthmatic.”

The study analyzed 941 of over 3,000 women recruited from private practices and hospital clinics in Connecticut and southern Massachusetts between 1997 and 2000. Following a baseline test early in the pregnancy, Kwon said, data were collected at two subsequent points before delivery through interviews and lung function tests. The participants were given hand-held machines that measured their lung function and stored the data for analysis by researchers.

Following the data-collection stage, standard multivariate statistical techniques were employed to correct for any possible other factors that might influence the data, such as environmental conditions, Bracken said.

The study expands on prior research establishing a correlation between fetal sex and the severity of maternal asthma.

“This is by far the largest study, so it’s the most reliable and it confirms some of the earlier work that there was some uncertainty about,” Bracken said.

The new study’s method was also different from that of past studies, he said, as the women were followed throughout the course of their pregnancies instead of only being evaluated once.

Kwon said the study suggests that sex-specific hormones given off by the fetus might impact asthmatic symptoms positively or negatively. But she said more information needs to be collected to verify the study’s hypothesis.

Asthma can also have negative effects on the fetus, resulting in pre-term delivery, low birth weight or miscarriage.

Future research will likely focus on the genetics of asthma, identifying the genes that cause it and how those genes interact with the environment, Bracken said.

“We are actually doing a whole range of studies looking at asthma, and this particular study was part of a project looking at asthma during pregnancy,” said Kathleen Belanger, deputy director of the Yale Center for Perinatal, Pediatric and Environmental Epidemiology. “We’re actually following up with the children of those pregnancies to look at respiratory development in children with mothers with asthma. … This is the early in utero piece of a much larger project that we’re conducting looking at asthma.”

The researchers stressed that though the results indicate that the sex of a fetus influences asthma, other factors — such as smoking and not treating the disease with proper medication — can have an even larger effect.

“There are still a lot of questions about what causes asthma,” Kwon said, “because it’s such a prevalent disease.”