Why is it bad to smoke while pregnant?

A New Study Details the Effects of Smoking in Pregnancy – TIME Healthland: The review, published Monday in the journal Human Reproduction Update from the European Society of Human Reproduction and Embryology, revealed a strong link between smoking and oral facial abnormalities that can arise when a flat embryo becomes a tube in the third week of pregnancy. It's a precarious time for an embryo, and smoking can disturb the process. Congenital heart disease is also distinctly associated with moms who smoke.