Women often develop rheumatoid arthritis at times when sex hormones are changing, such as the time after pregnancy or around menopause, although it's not known why. Researchers used a large, long-term study to find out whether hormonal factors affected women's chances of developing rheumatoid arthritis.

What the researchers wanted to know: What is the relationship between the start of rheumatoid arthritis and factors such as postmenopausal hormone use and the woman's age at her first birth?

What they did: Researchers used the Nurses' Health Study, a long-term look at 121,700 female nurses who were ages 30 to 55 when the study started in 1976. The study has spawned about a zillion papers looking at the risk factors for various diseases. For this paper, researchers looked at information on 674 women with rheumatoid arthritis, and compared them with about 104,000 women without rheumatoid arthritis. Every woman in the study fills out a questionnaire every two years.

What they found: Women who breast-fed had a slightly lower risk of rheumatoid arthritis, and it appeared that the longer they did so, the lower the risk. Women who had very irregular menstrual cycles had a somewhat higher risk of rheumatoid arthritis. They found no effect of taking oral contraceptives or postmenopausal hormones.

What the study means to you: Neither breast-feeding nor having irregular menstrual cycles probably alters risk enough for women and their doctors to worry about, but it's interesting that female hormones apparently relate somehow to rheumatoid arthritis.

Caveats: In many cases, women were trying to recall irregular periods and duration of breast-feeding long after the fact, so their memories may have been inaccurate.

Find out more: The National Institute of Arthritis and Musculoskeletal and Skin Diseases has information on rheumatoid arthritis.

Read the article: Karlson, E.W. et al. "Do Breast-Feeding and Other Reproductive Factors Influence Future Risk of Rheumatoid Arthritis?" Arthritis & Rheumatism. November 2004, Vol. 50, No. 11, pp. 3458–3467.

Abstract online: www3.interscience.wiley.com