Pregnant women with rheumatoid arthritis may have increased risks of high blood pressure, having an underweight baby or needing a cesarean section, a new study suggests.

Rheumatoid arthritis (RA) arises when the immune system mistakenly attacks tissue in the joints, leading to inflammation, pain and progressive joint damage. The disease is more common in women than men, and frequently develops during the childbearing years.

So far, studies have come to conflicting findings as to the potential effects of RA on pregnancy. Some, for example, have found that women with RA have higher risks of preterm delivery and having an underweight newborn, while others have found no such link.

For the new study, researchers used records from Taiwan’s national health system to compare 1,912 new mothers with RA with 9,560 new mothers without the disease.

They found that women with RA had a two-fold higher risk of pre-eclampsia - a potentially dangerous condition, marked by high blood pressure and protein in the urine, that develops in the second or third trimester.

Women with RA were also 47 percent more likely to have a low-birth-weight baby and 19 percent more likely to require a C-section, according to findings published in the Annals of Rheumatic Diseases.

Still, with the exception of C-section - reported for 42 percent of women with RA and 38 percent of those without RA - the large majority of women did not have these pregnancy complications.

Just under 3 percent of women with RA developed pre-eclampsia, compared with just over 1 percent of women in the comparison group. Eight percent of new moms with RA had a baby weighing less than 5.5 pounds, versus 5.5 percent of the comparison group.

Women with RA were also more likely to have a newborn who was “small for gestational age,” a sign of restricted growth in the womb. The problem was seen in 17 percent of women with RA, and 15 percent of women without the condition.

It is not clear why there is an association between RA and certain problems of pregnancy, according to Dr. Herng-Ching Lin and colleagues at Taipei Medical University.

Although the current study was large and allowed the researchers to account for a number of factors in the odds of pregnancy complications - like the women’s age and family income - it also lacked some important information.

The researchers had no information on the severity of each woman’s RA or medication use during pregnancy. So it’s not possible to tell how those factors might have affected the odds of complications, Lin’s team notes.

A number of RA medications, like methotrexate and leflunomide, may be harmful to the fetus and must be stopped before a woman conceives. But certain other medications, like prednisone and non-steroidal anti-inflammatory drugs such as ibuprofen, may still be used during pregnancy.

Future studies, Lin’s team writes, should try to determine the roles of RA severity and medication use in the pregnancy complications seen in this study. For now, the findings reinforce the recommendation that women with RA get good prenatal care, with regular visits to their obstetrician and rheumatologist.

SOURCE: Annals of Rheumatic Diseases, online February 25, 2010.