Many women with rheumatoid arthritis, a disorder in which your immune system mistakenly attacks your body's tissues, report improvement in their symptoms during pregnancy. Many also report a flare-up of symptoms after childbirth, usually within the first three months.

Researchers are studying why these changes occur. Because women are more likely than men to develop rheumatoid arthritis, one theory is that female sex hormones, estrogen and progesterone, play a role.

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

During pregnancy, the mother's immune system changes to prevent the rejection of the fetus. Researchers are studying whether these changes might be connected to an improvement in rheumatoid arthritis symptoms.