Recent research has confirmed that women who take, or have ever taken, the birth control pill are less likely to develop rheumatoid arthritis.
The new study - led by the Karolinska Institute in Stockholm, Sweden, and published in the journal Annals of the Rheumatic Diseases - also found that taking the birth control pill for 7 years or more was linked to the lowest risk of developing rheumatoid arthritis.
The researchers also discovered no protective effect from breast-feeding, after taking into account other factors that might have an influence.
Rheumatoid arthritis is an autoimmune disease - that is, a condition in which the immune system mistakenly attacks healthy tissue to cause inflammation, swelling, and pain. The disease mainly attacks the joints, most often the knees, hands, and wrists. However, it can also affect the lungs, heart, eyes, and other parts of the body.
When it attacks the joints, rheumatoid arthritis inflames the lining. Persistent inflammation gradually erodes the joint lining and tissue, causing chronic pain, deformity, and unsteadiness.
The disease affects up to 1 percent of people worldwide. Many become so disabled that it severely affects their quality of life. Within 10 years of onset, around half of rheumatoid arthritis patients in developed countries cannot maintain a full-time job.
Women more likely to have it than men
As yet, there is no cure for rheumatoid arthritis and nobody knows exactly what causes it. However, scientists have discovered that some factors can affect the risk of developing it.
Anyone can get the disease at any age, but the chances of getting it increase with age and women are typically two to three times more likely to be affected than men.
The researchers behind the new study note that while "hormonal and reproductive factors" are often given as an explanation for the sex differences in incidence of rheumatoid arthritis, the evidence to support it is mixed.
Some studies that have looked at oral contraceptive use and rheumatoid arthritis have shown that longer use of the pill is linked to lower risk of the disease. However, "the majority of reports have been unable to demonstrate an association," note the authors.
Evidence to support the idea that breast-feeding can reduce risk of developing rheumatoid arthritis is also mixed. Some studies have shown that it is linked to reduced risk, with longer breast-feeding having the strongest effect. However, other studies have found an increased risk.
The authors highlight that one problem with the previous evidence is that many of the studies did not test for potential blood markers of rheumatoid arthritis. They note that "few reports have taken seropositivity into account."
Blood markers tested
For the new study, the team examined the link between development of rheumatoid arthritis and use of the contraceptive pill, breast-feeding among adult women who had given birth to at least one child, or both.
The data that they used covered women aged 18 and over who were living in certain parts of Sweden between 1996 and 2014. It came from the Swedish Epidemiological Investigation of Rheumatoid Arthritis (EIRA), which is based at the Karolinska Institute.
Altogether, they studied 2,578 women with rheumatoid arthritis and 4,129 controls, which were age-matched women without the disease who had been randomly selected from the general population.
All participants had given blood samples, so the researchers could check for a blood marker called anti-citrullinated protein antibodies (ACPAs).
ACPAs are used as a blood marker for rheumatoid arthritis because 90 percent of people found to have them also have the disease. Their presence may also be a sign of a serious form of the disease.
The EIRA records also included comprehensive data about the women's reproductive histories (including use of contraceptives), lifestyles, whether they had breast-fed their children or not, and other factors such as level of education.
The records showed that 884 of the women with rheumatoid arthritis and 1,949 of the controls had breast-fed at least one child during the period between 2006 and 2014.
When they analyzed the data, the researchers found that, compared with never having used it, use of the contraceptive pill at any time was linked to a reduced risk of developing rheumatoid arthritis.
Current use of the pill was linked to 15 percent reduced risk, while past use was linked to 13 percent reduced risk.
The link was particularly significant among women who tested positive for ACPAs and remained so even after taking out the effect of other risk factors, such as smoking and alcohol, note the researchers.
The researchers suggest that there could be a "dose-response" effect: using oral contraception for more than 7 years was linked to a 19 percent lower risk of developing rheumatoid arthritis, even for women who tested negative for ACPAs.
The researchers note that 7 years was the average length of time for use of the contraceptive pill in the women they studied.
The analysis did reveal a lower chance of developing rheumatoid arthritis among women who had breast-fed, but this was not statistically significant once they took into account other risk factors.
The authors point out that their study was not designed to test cause and effect, so they could not draw any firm conclusions about whether use of oral contraception actually reduces the risk of rheumatoid arthritis or not.
However, a strong feature of the study was its large size and the fact that it took into account a wide range of other risk factors that might influence links to the disease.
"In this large population-based case-control study of incident RA [rheumatoid arthritis], with careful matching between cases and controls and extensive exposure information, we found that women who had ever used OCs [oral contraceptives] had a significantly decreased risk of developing RA."