Objective: To evaluate the impact of smoking and number of previous births on maternal serum levels of α-fetoprotein and free β-subunit of human chorionic gonadotropin (free β-hCG). Methods: The study included 3,252 completed unaffected singleton pregnancies that proceeded beyond 37 weeks’ gestation and resulted with a birth of healthy child. Smoking status of mothers and data concerning gravidity and parity were collected at the sampling date. Serum markers were measured between 13 and 22 gestational weeks, corrected for maternal weight, and converted to multiples of median (MoM) for unaffected pregnancy of the corresponding gestational age. Median MoM values for both markers were examined in relation to both: smoking habits and number of previous births. Results: Smokers had significantly decreased free β-hCG MoM values compared to nonsmokers (p < 0.001). The median levels showed a negative relationship with the number of previous births. The significance of a decreasing trend was proved, both in smokers (p < 0.001) and nonsmokers (p < 0.001). The median maternal serum α-fetoprotein MoM values did not show any significant dependence, neither with regard to smoking (p = 0.65) nor with regard to parity (p = 0.07). Conclusions: The recommendable adjustment of serum markers to smoking habits, especially concerning the free β-hCG levels, would be worthwhile. The evidence of the coexisting influence of parity on serum levels of free β-hCG, both in smokers and nonsmokers, should perhaps be a stimulus for reconsideration of which corrections the screening performance is dependent on.