Objective: It is known that onset of rheumatoid arthritis (RA) is increased post partum. We wanted to compare incidence rates between RA and other chronic arthritides (OCA) 0-24 months after delivery, and to compare the incidence rates within each group 0-24 versus 25-48 months post partum.
Methods: Premenopausal women from a Norwegian patient register were linked with the Medical Birth Registry of Norway to study the interval between delivery and time of diagnosis. Cox regression analysis with adjustments for age at delivery and birth order was applied to compare proportions of incident cases of RA and OCA with onset 0-24 months post partum. Poisson regression analysis with adjustment for population at risk was applied to estimate incidence rate ratio (IRR) 0-24 versus 25-48 months post partum.
Results: Out of 183 RA and 110 OCA patients diagnosed after delivery, 69 (37.7 %) had RA and 31 (28.2 %) OCA during the first 24 months post partum (p=0.09). The IRR (95 % CI) for diagnosis during 0-24 months versus 25-48 months was 1.73 (1.11, 2.70) (p=0.01) for RA, 1.05 (0.59, 1.84) (p=0.86) for OCA. The IRR was 2.23 (1.06, 4.70) and 1.87 (0.67, 5.21), respectively, when only considering diagnoses after 1st pregnancy. Clinical characteristics were similar within each diagnostic group.
Conclusion: The proportions of incident cases with onset 0-24 months after delivery were not different between RA and OCA. A peak in incidence during 0-24 months was seen in the RA group, both when considering all pregnancies and only the first pregnancy.