Mothers with rheumatoid arthritis are more likely to give birth prematurely and have babies of a lower birth weight than mothers without the condition. This is according to a new study published in the journal Arthritis & Rheumatology.

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Researchers found that children born to mothers with rheumatoid arthritis were up to 1.5 times more likely to be delivered preterm.

Rheumatoid arthritis (RA) is an autoimmune condition that causes pain, stiffness and inflammation in the joints – most commonly in those of the hands, feet and wrists.

The most recent figures from the Centers for Disease Control and Prevention (CDC) state that more than 1.5 million Americans have RA, and the disease is around three times more common among women than men.

According to the research team, led by Ane Rom of Copenhagen University Hospital in Denmark, past studies have associated rheumatic diseases with a number of pregnancy complications, such as preterm birth and low birth weight.

“Only a few studies investigated pregnancy outcomes among mothers diagnosed with RA specifically,” the researchers note, “and little is known about the importance of preclinical RA on pregnancy outcome.”

For their study, Rom and colleagues identified 1,917,723 singleton births that occurred in Denmark between 1977 and 2008 using data from national registries, including the Civil Registration System, the Medical Birth Registry and the Danish National Hospital Registry.

Of the children identified, 13,566 were born to mothers with either maternal RA (diagnosis received before giving birth) or preclinical RA (diagnosis received after giving birth).

The researchers found that, compared with children born to mothers without RA, those born to mothers with maternal RA were 1.48 times more likely to be delivered preterm, while those born to mothers with preclinical RA were 1.32 times more likely to be born prematurely.

On analyzing the birth weight of children born to mothers with maternal RA, the team found that they were around 87 g lighter than children of mothers without RA, while the placenta weight was around 14 g lower. This may be explained by the fact that these children were more likely to be born prematurely, the researchers say.

Children born to mothers with preclinical RA were also found to be of a lower weight at birth, compared with those born to mothers free of RA.

Commenting on the potential reasons behind their findings, the researchers say:

We hypothesize that maternal RA may affect fetal growth either through fetal programming related to the effect RA may have on the intrauterine environment, through genetic factors or due to medications taken through pregnancy.

The impact on fetal growth associated with maternal RA or preclinical RA could be caused by genetic factors rather than disease components affecting the intrauterine environment.”

Although the team says the association between RA, preterm birth and low birth weight warrants further investigation, Rom believes obstetricians “should be aware of the increased risk of preterm birth in women with RA and among those with preclinical signs of the disease.”

The researchers stress, however, that the reduction in fetal growth of children born to mothers with RA was only small, and it is unlikely to have any impact on the child’s perinatal conditions. “Whether it has long-term health consequences for children of mothers with RA is unknown and needs to be studied,” they add.

A recent study reported by Medical News Today suggests that children born preterm and of a low birth weight may be at higher risk of needing a hip replacement in adulthood.