Women who breast-feed for longer have a smaller chance of getting rheumatoid arthritis, according to a new study.

The study also found that taking oral contraceptives, which are suspected to protect against the disease because they contain hormones that are raised in pregnancy, did not have the same effect.

Also, simply having children and not breast-feeding did not seem to be protective, according to a news release.

The researchers compared 136 women with rheumatoid arthritis with 544 women of a similar age without the disease. They found that those who had breast-fed for longer were much less likely to get rheumatoid arthritis.

Women who had breast-fed for 13 months or more were half as likely to get rheumatoid arthritis as those who had never breast-fed. Those who had breast-fed for one to 12 months were 25 percent less likely to get the disease.

The proportion of women breast-feeding for more than six months has increased dramatically over the past 30 years. The authors concluded that it was difficult to say whether there was a connection between higher rates of breast-feeding and a corresponding fall in the number of women affected by rheumatoid arthritis, but that the results of the study provided yet another reason why women should continue breast-feeding.

The work will appear in the Annals of the Rheumatic Diseases.