Smoking and pregnancy

The adverse effects of smoking begin even before birth. Maternal smoking reduces fertility and adversely affects pregnancy outcomes. Smoking during pregnancy reduces birth weight by approximately 200 grams on average. The degree of birth weight reduction is related to the amount smoked. If a mother who smokes gives up this behaviour by the third trimester of pregnancy, much of the weight reduction can be avoided. Smoking also increases rates of other adverse effects on reproduction including spontaneous abortion, and smoking during pregnancy is now considered to be a cause of sudden infant death syndrome. There is more limited evidence suggesting that smoking by the mother while pregnancy may increase risk for congenital defects, especially cleft lip and palate.
Smoking during pregnancy also causes acute lower respiratory tract infections; asthma induction and exacerbation; chronic respiratory symptoms; middle ear infection.
Smoking while pregnant has been shown to reduce the production of a chemical that relaxes blood vessels and can lead to lowered blood flow to the fetus. It is resulting in lower birth weight, shorter length and smaller head circumference.
Researchers also found that if the expectant mother quits smoking early in pregnancy, the enzyme that produces nitric oxide returns to normal levels and the infant is born at normal birth weight.