According to this study:

* Infants of women who smoked one month before pregnancy or during the first trimester had an increased risk of congenital heart defects.

Congenital heart defects were more common in infants whose mothers smoked in the month prior to or the first trimester of pregnancy than in those whose mothers didn't smoke during these times, according to the results of a population-based case-control study.

Researchers studied 3,067 infants with nonsyndromic congenital heart defects and 3,947 infants without birth defects who were entered in the National Birth Defects Prevention Study. Maternal smoking and passive exposure to tobacco smoke in the home or workplace during the periconceptional period (defined as one month prior to pregnancy through the first trimester) were assessed. Smokers were categorized as light (up to 14 cigarettes per day), medium (15 to 24 cigarettes per day), or heavy (25 or more cigarettes per day).

Infants with congenital heart defects were 3.92 times more likely to be of low birth weight than control infants. Infants with septal heart defects were more commonly borne by women who reported smoking during the periconceptional period than by those who didn't smoke during that period. The risk of septal defects increased as the amount of smoking increased, with heavy smokers' infants having a doubled risk compared with those of light smokers. Heavy smoking was also associated with right-sided obstructive defects. There was no significant association between environmental smoke exposure and congenital heart defects.

More than a quarter of U.S. women of childbearing age smoke, and approximately 20% of them continue to smoke after becoming pregnant. Nurses need to advise women not to smoke during this time and to offer them assistance in quitting.