By Robert Preidt
For the study, researchers looked at data from almost 2 million single-baby births in Denmark between 1977 and 2008. They found that more than 13,500 of the mothers had rheumatoid arthritis or were diagnosed with the disease after giving birth ("preclinical" rheumatoid arthritis).
The odds that women with the disease would have a premature baby were 1.5 times higher than for those without the condition, the study found. For women with preclinical rheumatoid arthritis, the odds of a premature delivery were 1.3 times higher.
The researchers also linked slightly lower birth weights to infants born to mothers with either diagnosed or preclinical disease than those born to mothers without the illness.
However, this study was only designed to find an association between rheumatoid arthritis and premature birth or low birth weight. The study wasn't designed to prove that the illness caused those problems.
Rheumatoid arthritis in fathers had no effect on the risk of premature birth or the baby's weight, according to the study published Nov. 13 in the journal Arthritis & Rheumatology.
"Obstetricians should be aware of the increased risk of preterm birth in women with rheumatoid arthritis and among those with preclinical signs of the disease," study leader Ane Rom, of Copenhagen University Hospital, said in a journal news release.
"For women with rheumatoid arthritis, we found only a small reduction in fetal growth in their babies, which has little impact on the children immediately following birth. The long-term health effects for children born to mothers with rheumatoid arthritis need further investigation," Rom added.
About 1 percent of people worldwide have rheumatoid arthritis, the researchers noted. The condition affects three times as many women as men, according to the Arthritis Foundation.