Background: Two previous reports suggest that male sex and maternal smoking reduce TWBC and TNC in cord blood samples. Objective: To assess whether the following factors: maternal smoking at term, male sex, maternal group B streptococcal colonization (GBS-c) or spontaneous onset of labor/induction of labor affect TWBC and TNC in neonates in the first 24 hrs of life. Study design: Cross sectional cohort study. Subjects: 2550 healthy infants born to healthy mothers with prelabor rupture of the membranes at term, with known GBS-c and smoking status, who were enrolled in a multicenter trial of induction of labor vs. conservative management. Methods: Blood samples for TWBC, TNC and bacterial cultures (with negative results) were obtained within 24 hrs of birth. Statistical analysis: regression models adjusting for postnatal age at time of sampling were used to determine the effect of the factors on TWBC and TNC. Results: Mode of onset of labor and GBS-c did not statistically significantly change the indices. Statistically significant reductions (p<0.0001) in TWBC and TNC were noted in male infants and in infants born to mothers who smoked. An additional statistically significant reduction in TWBC (p=0.04) was noted in males due to an interaction between male sex and maternal smoking with a similar trend for TNC(p=0.11). Table Conclusions: Mode of onset of labor and GBS-c do not affect TWBC/TNC in healthy neonates. TWBC and TNC are lower in male than female neonates. Maternal smoking reduces TWBC/TNC in both sexes; the reduction is more pronounced in male neonates. (Funded by Medical Research Council of Canada).

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