They found that MS patients who nursed their babies exclusively - meaning no bottled formula - for at least 2 months appeared less likely to have a relapse within a year of the child's birth than women who did not breastfeed.
"It is well-known that women with MS have fewer relapses during pregnancy and a high risk of relapse in the postpartum period," the researchers wrote in the Archives of Neurology.
Women also are advised not to take MS drugs during pregnancy or while breastfeeding, so patients must choose between nursing their babies and resuming treatment. The American Academy of Pediatrics recommends breastfeeding exclusively for the first 6 months of a baby's life and continued nursing for at least a year.
Dr. Annette Langer-Gould, formerly of Stanford University School of Medicine in California and now of Kaiser Permanente Southern California in Pasadena, and her colleagues studied 32 pregnant women with MS and 29 pregnant women without MS.
Nearly all - 96% - of the healthy women breastfed their babies, as opposed to 69% of the MS patients.
They found that 87% of the women with MS who did not breastfeed, or who started using formula within 2 months, had a relapse, compared to 36% of those who gave their babies only breast milk for at least 2 months.
The women who breastfed exclusively delayed the return of normal menstruation, the researchers noted. Those whose monthly cycles stayed repressed, a normal effect of breastfeeding, were those whose MS symptoms did not return.
"Studies of immunity and breastfeeding, while plentiful, are predominantly focused on breast milk content and health benefits to the infant. Little is known about maternal immunity during breastfeeding," the researchers wrote.
Multiple sclerosis occurs when the immune system mistakenly attacks the fatty myelin sheath protecting nerve cells. It affects 2.5 million people globally - disproportionately women of childbearing age. It can cause mild illness in some people and permanent disability in others.
Symptoms may include numbness or weakness in the limbs, loss of vision and an unsteady gait.
Do you have MS? Have you experienced this?