March 7, 2012 -- Pregnancy appears to play a strong role in whether or not a woman may develop the autoimmune disease multiple sclerosis (MS), according to a new study.

The study involved more than 800 women between the ages of 18 and 60. Nearly 300 of them had experienced a first episode of MS symptoms. The other women were healthy and were included for comparison.

Women in the study with at least one child had about half the risk of early MS symptoms compared to women without children. And that risk appeared to drop with each additional child. Women with three children had a 75% lower risk of early MS symptoms compared to women without children. In women with five or more children, risk of early symptoms was slashed by 94%.

Those benefits remained even after researchers accounted for other factors associated with the likelihood of developing MS, like level of education, smoking, skin damage and sun exposure, and certain susceptibility genes.

Researchers say they’re pretty sure that it is something about pregnancy -- rather than being a parent or raising children -- that’s protective, because they saw no difference in men.

The study is published in the journal Neurology.

Slideshow: A Visual Guide to Multiple Sclerosis
Pregnancy and Early MS

Although it is already known that a woman with MS may see a decrease in her symptoms while pregnant, other large studies have not seen an association between pregnancies and MS. But researchers think that may have something to do with when women were included in the studies.

In the current study, women were enrolled after their first episode of MS symptoms.

“It might have been blurred vision or a funny leg, and they are found to have [nerve damage] on an MRI scan,” says researcher Anne-Louise Ponsonby, PhD, a professor at the Murdoch Children’s Research Institute in Melbourne, Australia.

Ponsonby says about two-thirds of people who experience early symptoms will go on to develop full-blown multiple sclerosis, a disease in which the body slowly attacks its own nerve cells. The damage eats away at the protective coating around nerves, disrupting nerve signals. This disruption causes myriad symptoms, including trouble with movement, balance, coordination, vision, and speech.

Catching women when they were first experiencing symptoms was important, Ponsonby says, because researchers were able to look at the relationship between pregnancy and MS symptoms before the disease had influenced a woman’s choice to have children.

Many young women diagnosed with MS choose not to become pregnant because of fears about whether they’ll be able to care for their children.

Previous studies had not seen an association between pregnancy and MS, perhaps because of this bias.

“This is to my knowledge the first high-quality study ... which suggests a beneficial effect of pregnancy,” Martin Daumer, PhD, scientific director of the Sylvia Lawry Centre for MS Research in Munich, Germany, says in an email. Daumer wrote an editorial on the study but was not involved in the research.

Researchers say they aren’t sure what it is about pregnancy that may be protective, but they have some theories.

“Multiple sclerosis is a disease that’s caused by immune system overactivity and irritability,” Ponsonby says. “When you’re pregnant, your body gets trained into being very, very tolerant. That’s how the immune system doesn’t reject the baby. So you get this very big training on being able to handle something that’s foreign, or not exactly ‘self,’ in the body.”

Another idea is that cells that are shed by the baby, called fetal cells, stay in mom’s body and may lead to long-term changes in the way her immune system works.

Experts say if further research confirms the findings, they may help explain the rising incidence of MS in women.

“It is possible that differences in the patterns with which people are approaching marriage, pregnancy, children, and when they have children could be affecting this change in incidence,” says Nicholas LaRocca, PhD, vice president for health care delivery and policy research at the New York City-based National Multiple Sclerosis Society, the organization that funded the research.

“If there is this protective effect of pregnancy, as the study suggests, then to some extent in industrialized countries, we’ve begun to abandon some of this protection,” LaRocca says.