To determine if protective behavior prevents child-to-mother transmission of cytomegalovirus (CMV) during pregnancy.

We studied 166 seronegative mothers (94% white women; mean age, 33 years) with a child <36 months of age attending a day care facility. Mothers, either pregnant or attempting pregnancy, were randomly assigned by day care center to either a control or intervention group. Mothers in the intervention group received instructions for hand washing, glove use, and for avoiding types of intimate contact with their child. The control group received no instructions or information about their serologic status or whether their child was shedding CMV.

In the intervention group, 7.8% of women (9 of 115) seroconverted, as did 7.8% of women (4 of 51) in the control group. Two independent predictors of maternal infection were (1) a child shedding and (2) a mother attempting pregnancy at enrollment. For 41 women attempting pregnancy at enrollment with a child shedding CMV, 10 of 24 became infected compared with only 1 of 17 women who were already pregnant at enrollment (P=.008).

For seronegative women who already know they are pregnant, intervention may be highly effective for preventing CMV acquisition.