Objective. To determine the outcome of pregnancy in women with rheumatoid arthritis (RA) in relation to disease activity and medication use during the pregnancy.Methods. In a prospective study, pregnant women with RA were evaluated before conception (when possible), during each trimester of the pregnancy, and postpartum. Clinical characteristics, disease activity, medication use, and pregnancy outcome were analyzed. To examine the independent influence of prednisone use and disease activity on birth weight, regression analyses were performed, with adjustments for gestational age of the child at delivery, the sex of the newborn, and the mother's smoking status, education level, parity, and use of an assisted reproduction technique. KaplanMeier curve analyses were performed to examine the association between medication use and gestational age at delivery.Results. Data from 152 Caucasian RA patients with singleton pregnancies were available. Both the mean ؎ SD birth weight (3,379 ؎ 564 gm) and the mean ؎ SD birth weight standard deviation score (SDS; ؉0.1 ؎ 1.1), which is the birth weight adjusted for the gestational age and sex of the newborn, were comparable with those in the general population. On multiple linear regression analyses of birth weight and birth weight SDS, both of which were adjusted for covariates, only disease activity was associated with lower birth weight (P ‫؍‬ 0.025). The gestational age at delivery was significantly lower in women who were taking prednisone (38.8 versus 39.9 weeks; P ‫؍‬ 0.001), and their delivery was more often premature (<37 weeks; P ‫؍‬ 0.004). Conclusion. Pregnancy outcome in women with well-controlled RA is comparable with that in the general population. The effect of prednisone on birth weight is mediated by a lower gestational age at delivery, whereas a higher level of disease activity independently influences birth weight negatively, suggesting an immune-mediated mechanism.