Pregnancy

In the first half of the 20th century, some women with multiple sclerosis (MS) were advised by their neurologist not to become pregnant, or even to have a therapeutic abortion if they did.¹ However, now it is widely accepted that relapse rate is reduced during pregnancy, at least in the second and third trimester. Furthermore, there is no evidence that pregnancy is associated with negative long-term consequences and some studies have actually suggested beneficial effects.^2,3

Studies of the effect of pregnancy on MS disease course are confounded by effects of disease severity on the decision of whether to become pregnant. Those with less severe disease reasonably might be expected to choose to become pregnant more often than those with more severe disease due to the fear of not being able take care of a child. An elegant approach to circumventing this confound was undertaken in the study by Ponsonby and colleagues⁴ in this issue of Neurology® by confining their analysis to persons with a first demyelinating event. In this study of 282 Australian cases and 542 controls, recruited from several sites, they found that the number of pregnancies was inversely associated with the risk of having a first demyelinating event. They concluded that this was consistent with a cumulative effect of pregnancy to reduce the risk of developing MS to one-fourth of the nulliparous risk for those with 3 or more pregnancies, and to one-20th for those with 5 or more pregnancies. Although case-control studies are prone to confounding, the authors considered a number of genetic and environmental confounders, including HLA type and sun exposure, among others, and none seemed to explain the observations.

Another important conclusion from this work was that prenatal biologic factors in the mother, not postnatal environmental factors that would …