Rheumatoid arthritis (RA) can affect anyone—male or female, young or old. Still, the fact remains that women are two to three times more likely to have RA than men. What's more, recent research in the journal Arthritis & Rheumatism reports that after 40 years of decreasing, the rate of RA in women has begun to creep upward. In the study, researchers examined the health records of 1,761 men and women who were diagnosed with various forms of arthritis between 1995 and 2007. They found that the incidence of RA increased by 2.5 percent per year in women during that period, but there was no rise among men. Why the uptick in RA among women? While experts aren't sure, they offer three theories: Changes in oral contraceptives. Previous research found that oral contraceptives protected women against RA. However, current birth control pills contain lower doses of synthetic estrogens than those that were popular several decades ago. Newer contraceptive devices also introduce lower amounts of estrogen into the bloodstream. These lower levels of synthetic estrogens may provide less protection against the disease. Cigarette smoking. Smoking is one of the strongest risk factors for developing RA in both men and women. While the habit has become increasingly unpopular throughout the U.S., smoking rates are declining more slowly among women compared with men. This could help explain why the rate of RA in women hasn't diminished. Vitamin D deficiency. Studies suggest that not getting enough vitamin D may put people at risk of developing RA. Meanwhile, the rate of vitamin D deficiency has increased among women in recent years, which could explain why they are still at a higher risk of developing RA than men. Scientists believe that the increase in RA rates among women may not be due to a single factor. Instead, a combination of issues—such as those above and potentially others—may heighten women's risk for RA while also decreasing their protection against the disease. Key Takeaways Women are two to three times more likely to have RA than men. Recent research reports that after 40 years of decreasing, the rate of RA in women has begun to creep upward. Changes in oral contraceptives, cigarette smoking, vitamin D deficiency, or some combination of factors may explain the increase.