Women with rheumatoid arthritis who are pregnant tend to deliver prematurely and their babies often have low weight. That’s according to new research presented June 14 in Amsterdam at the Annual European Congress of Rheumatology.

The study of 845 pregnant women with rheumatoid arthritis defined low birthweight as less than 2.5 kg, and it defined premature birth as prior to 37 weeks. The mothers were selected between 2001 and 2012 from among 2 million in the Taiwan National Health Insurance database and birth registry.

It is well known that many pregnant women with rheumatoid arthritis see improvements in their symptoms, which is thought to be due to the body suppressing its immune system to avoid rejecting the fetus. But the impact of rheumatoid arthritis on the pregnancy was understudied, according to a EULAR news release.

“Our results add to a growing body of evidence from different populations suggesting small
but significant increases in prematurity and a decrease in birth weight in pregnancies in
mothers with rheumatoid arthritis,” said study author Yun-Chen Tsai, of Taiwan’s Chang Gung Memorial Hospital in the release. “While these findings are important, they should not discourage women with RA from trying to conceive.”

Tsai and colleagues found that moms with rheumatoid arthritis were 1.65 times likelier to give birth to a light baby, about 1.4 times likelier to deliver prematurely, and about 1.6 times likelier to have a baby that was small for its gestational age. But when investigators studied whether there were risks to the mother, they didn’t find any.