Maternal cigarette smoking in the first trimester was associated with a 20 to 70 percent greater likelihood that a baby would be born with certain types of congenital heart defects,according to a CDC study. The study found an association between tobacco exposure and certain types of defects such as those that obstruct the flow of blood from the fair side of the heart into the lungs and openings between the upper chambers of the heart (atrial septal defects). Based on the findings of this and other studies, eliminating smoking before or very early in pregnancy could prevent as many as 100 cases of fair ventricular outflow tract obstructions and 700 cases of atrial septal defects each year in the United States.
Source: cdc.gov