A new study finds that women who give birth may have a lower risk of developing rheumatoid arthritis than women who remain childless - though the potential protective effect seems to fade over the years.

Rheumatoid arthritis (RA) arises when the immune system mistakenly attacks tissue in the joints, leading to inflammation, pain and progressive joint damage. The disease is more common in women than men, and frequently develops during the childbearing years.

Some studies, but not all, have suggested that pregnancy may lower a woman’s risk of developing RA. Exactly why having children would be protective is not clear, but one theory is that fetal cells transmitted to the mother during pregnancy help lower RA risk.

For the current study, researchers at the Fred Hutchinson Cancer Research Center and University of Washington in Seattle examined the pregnancy histories of 310 women who had been newly diagnosed with RA and 1,418 women without the disease.

Overall, women who had had at least one child were 39 percent less likely to have RA than women who had never been pregnant - with factors including age and oral contraceptive use, which has been linked to a lower RA risk, taken into account.

However, the protective relationship grew weaker over time, Dr. Katherine A. Guthrie and her colleagues report in the journal Arthritis & Rheumatism.

Women who had had their last child within the past 5 years were 71 percent less likely than childless women to have RA, whereas the risk reduction was 24 percent among women who had last given birth more than 15 years ago.

“The most exciting result for us,” Guthrie told Reuters Health in an email, “was the relationship of a woman’s risk for developing RA with time from childbirth - in particular diminishing protection over time - because this observation supports our hypothesis that fetal cells, now known to persist for decades after a birth, may benefit the mother.”

The researchers found that of the 120 women in the study who had given birth in the past five years, 9 percent had RA. That figure was 14 percent among the 345 women who had had a child in the past 5 to 15 years, and 17 percent among the 805 women who had given birth more than 15 years ago.

Among the 406 women who had remained childless, 24 percent had RA. However, because the study compared a group of women with RA to a healthy group, those percentages do not reflect a woman’s risk of developing RA based on pregnancy history. It’s estimated that 1.3 million U.S. adults, or about 0.6 percent of the adult population, have RA.

The current findings do not prove that having children lowers a woman’s risk of rheumatoid arthritis. But it’s possible, Guthrie explained, that fetal cells that persist in the mother’s body offer some protection against the disease.

Those fetal cells are genetically different from the mothers’ since half of a child’s genes come from the father. And if the cells carry genes associated with a decreased risk of RA, then that could, in theory, affect a woman’s likelihood of developing the disease, according to the researchers.

It also makes sense, Guthrie said, that such protection would wane over time, due to the “natural aging” of the fetal cells, and possibly also due to the aging of the woman’s immune system.

More studies are needed to look into the possibility that pregnancy confers some protection against RA, the researchers say. The answer to that question, according to Guthrie, could have broader implications for the understanding of how RA develops, and possibly point to ways to prevent it.

SOURCE: Arthritis & Rheumatism, online March 22, 2010.