Expectant mothers with rheumatoid arthritis (RA) are more likely to give birth prematurely, according to a recent study.

Researchers from Denmark and the United States found that babies of women with RA or pre-clinical RA -- the period prior to symptoms -- are 1.5 times more likely to be born prematurely in Denmark. The findings indicate that body measurements of the baby at birth were only slightly lower in children exposed to maternal or preclinical RA compared to those with no exposure to the disease.

For the study, researchers analyzed national registries to identify all children born in Denmark between 1977 and 2008. After excluding for multiple child births, children adopted, or those who could not be linked to their mother, researchers had a final study group of nearly 2 million children.

The findings show that 13,566 children were exposed to maternal RA (mother was diagnosed prior to giving birth) or preclinical RA (mother diagnosed after giving birth). Children (2,101) born to mothers with RA had similar measurements -- birth length, head and abdominal circumference -- at birth compared with babies of mothers without RA. In mothers with RA, birth weight of the baby was 87 grams (3oz) lower and placenta weight was 14 grams (0.5oz) lower than babies born to mothers without the disease. Researchers found rather similar results in children (11,455) exposed to preclinical RA.

Compared with unexposed children, researchers determined preterm birth risk was higher in children exposed to maternal RA and pre-clinical RA with an odds ratio of 1.48 and 1.32, respectively.

"Obstetricians should be aware of the increased risk of preterm birth in women with RA and among those with preclinical signs of the disease," Ane Rom, who led the study, said in a statement. "For women with RA, 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 RA need further investigation."

The findings are detailed in Arthritis & Rheumatology, a journal of the American College of Rheumatology.