A new study suggests that the age when a woman has her first child may be linked to her chance of developing rheumatoid arthritis later in life.
The pattern was not simply that younger or older was better. Instead, researchers found the lowest risk among women whose first birth occurred around ages 30 to 32.
Rheumatoid arthritis is a long-lasting disease in which the immune system mistakenly attacks healthy tissue, especially the lining of the joints.
It can cause pain, swelling, stiffness and difficulty moving, and over time it may damage joints. The condition affects women more often than men, which has led scientists to study whether reproductive and hormonal factors may play a role.
The research was carried out by scientists from the First Teaching Hospital of Tianjin University of Traditional Chinese Medicine in China. It was published on January 16, 2024, in the peer-reviewed journal PLOS ONE.
The team used information from the U.S. National Health and Nutrition Examination Survey, or NHANES, collected between 2011 and 2020.
The analysis included 7,449 women and focused on their age at first birth. Researchers compared this information with whether the women had rheumatoid arthritis. They also used statistical methods to account for other factors that could affect the results.
A clear U-shaped pattern appeared in the data. Women who had their first child at relatively young ages, as well as women whose first birth occurred after age 35, were more likely to have rheumatoid arthritis. Women who first gave birth between ages 30 and 32 had the lowest observed risk and were used as the comparison group.
The difference was especially large among the youngest mothers. Compared with women who first gave birth at ages 30 to 32, those who did so before age 18 had about four times the odds of rheumatoid arthritis. The odds were also roughly three times as high for women whose first birth occurred at ages 18 to 20, 21 to 23, or 24 to 26.
Women who had their first child after age 35 also showed higher odds, at about 3.4 times those of the 30-to-32 group. However, the study did not find a statistically clear increase among women who first gave birth at ages 27 to 29 or 33 to 35. This suggests that the relationship may be more complex than a simple effect of delaying pregnancy.
Scientists do not yet know why age at first birth might be connected with rheumatoid arthritis. Pregnancy causes major changes in hormones and in the immune system, and some of these changes can continue after childbirth. Social and health factors linked with very early or later motherhood could also contribute to the pattern.
Importantly, the study cannot prove that having a first child at a certain age causes rheumatoid arthritis. It was an observational study, meaning researchers looked for links in existing health data rather than assigning women to different pregnancy ages. Other factors that were not fully measured could help explain the findings.
The results may still help researchers better understand why rheumatoid arthritis develops and which women could benefit from closer attention to joint symptoms.
Persistent joint swelling, pain or morning stiffness should be discussed with a health professional, because early diagnosis and treatment can help limit joint damage. The researchers say more studies are needed to confirm the connection and uncover the biological or social reasons behind it.
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