Smoking decreases the amount of oxygen and increases the amount of carbon monoxide crossing the placental barrier. This is especially dangerous when a woman is pregnant with twins or more because of the increased oxygen demand due to multiple fetuses.

Smoking increases the probability of premature birth, low birth weight, still birth, and sudden infant death syndrome (SIDS). Mothers who continue to smoke during pregnancy place their children at significantly greater risk for depression and anxiety and increase the likelihood that the child will engage in early tobacco use.