General characteristics of the participants
Socio-demographic data of cases and controls are presented in Table 1. Maternal age was slightly higher in the control group as compared to infected mothers (26.2±5.1 vs. 25.8±4.7 years). The distribution of maternal employment status indicated a higher socioeconomic status of case mothers. The proportion of managerial mothers was 26.1% (N=585) in the group of cases and 19.2% (N=10,533) in the control group, while that of unskilled mothers were 1.9% (N=44) in the infected group, and 3.6% (N=1,967) in controls. No significant differences were found in birth order (primiparous women among cases: 1,276 (57.0%) vs. 48,801 (54.9%) in the control group). A highly significant difference between the two study groups has been found in maternal smoking. The proportion of smoker mothers was 15.9% in the group of viral infections while 8.7% in the control group.