Every cigarette contains 4,000 chemicals, which go into your lungs when you smoke. Once they've gone into your lungs, the nicotine, poisons and carbon monoxide cross the placenta to the baby.
Nicotine narrows your blood vessels, which reduces the blood flow in the placenta. Less blood means less oxygen and nutrients reach the baby and because of this they may not grow as well as expected. Babies born to smokers tend to weigh less at birth than babies born to non-smokers.