BOSTON (AP) _ Women with multiple sclerosis who get pregnant are likely to see their disease briefly slow and then speed up, but pregnancy does not change the long-term course of the disease, a study finds.

Doctors have long known that pregnancy can cause fluctuations in the progression of MS. However, the disease often flares up and then plateaus, and most studies have been too small to draw firm conclusions about how pregnancy affects this.

The latest study was conducted on 254 women in 12 European countries. Doctors looked at how their disease changed over a 33-month period before, during and after pregnancy.

Compared with the year before they got pregnant, there was a 70 percent decrease in the rate of relapse during the women's third trimester. But during the first three months after birth, there was a 70 percent increase in relapse over their pre-pregnancy rate.

Despite these blips, the doctors concluded that pregnancy had no overall effect on the rate of the progression of MS.

The study was conducted by Dr. Christian Confavreaux and others from l'Antiquaille Hospital in Lyons, France. It was published in Thursday's issue of the New England Journal of Medicine.

In a journal editorial, Dr. John N. Whitaker of the University of Alabama in Birmingham said the study means women with MS can be told that pregnancy or receiving anesthesia during delivery will not affect the course of their disease. But they should be warned that the risk of relapse immediately afterward may reduce their ability to care for their babies.

In the United States, between 250,000 and 350,000 people have MS. The disease is twice as common among women as men.

MS results from a misguided attack by the body's immune defenses on nerve fibers. Pregnancy probably influences MS by affecting the immune system.