You are more likely to give birth to a baby who weighs less than expected (low birth weight) if you smoke. This can lead to many medical problems for your child.
Smoking while pregnant also increases the risk of birth defects, such as cleft lip and cleft palate.1
Women who smoke also have a higher risk of problems with the placenta, which nourishes the unborn baby and removes its wastes. This can cause problems throughout your pregnancy and with your newborn.
If you quit smoking before you become pregnant (or sometime during the first 3 months of your pregnancy), your risk of having a baby with low birth weight is the same as that of a woman who does not smoke. Women who quit later in their pregnancy still reduce the risk of problems for their babies.
Citations
WebMD Medical Reference from Healthwise