Two of the four rotating warning labels required on cigarette packages in the U.S. mention the effects of smoking during pregnancy. One says "Smoking causes lung cancer, heart disease, emphysema, and may complicate pregnancy." The other says "Smoking by pregnant women may result in fetal injury, premature birth, and low birth weight."
The information on the labels is correct, but it's only a small part of the total picture of how smoking can affect your reproductive system and how smoking during pregnancy can harm both you and your baby.
When a pregnant woman smokes, so does her baby. The only difference is that the mother is exposed to tobacco smoke through her lungs, while the baby is exposed through the placenta (the afterbirth, the special organ that connects the mother and baby during pregnancy). Toxic substances from tobacco smoke travel through the mother's bloodstream to the placenta and from there to the baby's bloodstream. These substances interfere with the baby's ability to get enough of the oxygen and nutrients needed for normal growth and development.