Smoking during pregnancy does carry many risks with it. While there is a lot of conflicting data about the degree and type of effects, there are a few points that most studies will agree upon.
Smoking may increase the risk of preterm premature rupture of membranes, abruptio placentae, and placenta previa, and with a modest increase in risk for preterm delivery. Infants born to women who smoke during pregnancy have a lower average birth weight and may be more likely to be small for gestational age than are infants born to women who do not smoke. Smoking during pregnancy may also increase the risk of stillbirth, miscarriage, and severe vaginal bleeding.
Additonally, smoking can affect the mother during pregnancy. If you smoke while you are pregnant, you will have an increased rate of chest infections and blood clotting disorders.
Nursing mothers who smoke more than 20 cigarettes a day will likely pass along harmful chemicals from cigarettes to their babies in breast milk. Heavy smoking can reduce a mother's milk supply, and on rare occasions has caused symptoms in the breastfeeding baby such as nausea, vomiting, abdominal cramps, and diarrhea.
Studies have shown that these risks are drastically reduced when a woman quits smoking during pregnancy, particularly during the first trimester. At a minimum, a reduction in smoking habits will at least mildly reduce some of these risks.
Many aids are available for quitting smoking during pregnancy. The Nicotine patch has been shown in some studies to be safe for pregnant women. However, studies are ongoing and you should consult with your physician about using the patch while you are pregnant. Medications such as the anti-depressant Wellbutrin, which has shown to be an effective smoking cessation aid, may or may not be safe to use during pregnancy. Again, these should be used under a physician's care, and only when absolutely necessary.