Cigarette smoke contains more than 4,000 chemicals, including truly nasty things like cyanide, lead, and at least 60 cancer-causing compounds. When you smoke during pregnancy, that toxic brew gets into your bloodstream, your baby's only source of oxygen and nutrients.
The most serious complications — including stillbirth, premature delivery, and low birth weight — can be chalked up to the fact that nicotine and carbon monoxide work together to reduce your baby's supply of oxygen.