Rochester, Minnesota - Many women report a significant improvement in their rheumatoid arthritis symptoms during pregnancy and a flare-up of symptoms after childbirth, but experts aren't certain why this occurs.

Women are more likely than men to develop rheumatoid arthritis, so sex hormones may play a role in the disease. The main female sex hormone is estrogen.

But women who take medications containing estrogen - as part of their oral contraceptive or hormone replacement therapy for menopause - usually don't experience any change in their rheumatoid arthritis symptoms.

Researchers are studying whether changes in the body's immune system during pregnancy might be connected to an improvement in rheumatoid arthritis symptoms.

Rheumatoid arthritis is a disorder that occurs when your immune system mistakenly attacks your body's own tissues. During pregnancy, the mother's immune system changes to prevent the rejection of the fetus. This change might also reduce rheumatoid arthritis symptoms.