Risk Factors Of Smoking

Low birth weight, increased risk of birth defects, increased risk of miscarraige. Nicotine is a vasoconstrictor, it constricts blood vessels, reducing blood flow in your body. Smoking during pregnancy reduces the blood flow to the baby. Also nicotine crosses the placenta and may enter bloodstrem child, causing vasoconstriction in the system of child development. It is better to stop smoking if you are pregnant or if you intend to get pregnant.

The post above me has the most impact, even if I'm not sure about the birth defects. It might be on the packaging, but I've never seen a study showing that smoking causes birth defects. Drugs are not. Birth defects are genetic, not environmental.