Smoking during pregnancy restricts oxygen to the fetus, raises the risk of serious placental complications, and can permanently alter how a baby’s brain and lungs develop. About 5.4% of women with a recent live birth in the United States reported smoking during pregnancy in 2021, and the effects touch nearly every stage of fetal growth.
How Smoking Restricts Oxygen to the Fetus
Cigarette smoke delivers two substances that directly interfere with fetal oxygen supply. Carbon monoxide binds to red blood cells more aggressively than oxygen does, reducing how much oxygen your blood can carry to the placenta. Nicotine narrows blood vessels, including the ones feeding the uterus, which further cuts the flow of oxygen-rich blood reaching the baby. The combined effect creates a state of chronic low oxygen for the fetus throughout pregnancy.
This oxygen restriction is the root cause of many downstream problems. A fetus that consistently receives less oxygen and fewer nutrients grows more slowly, and the organs most sensitive to oxygen deprivation, particularly the brain and lungs, are the ones most affected.
Placental Complications
The placenta is especially vulnerable to the vascular damage smoking causes. Women who smoke have roughly twice the risk of placental abruption, a condition where the placenta separates from the uterine wall before delivery. Abruption can cause heavy bleeding and is a medical emergency that threatens both the mother and baby. The risk of placenta previa, where the placenta covers the cervix, also rises by about 36% in smokers compared to nonsmokers. Both conditions increase the likelihood of preterm delivery and cesarean birth.
Lower Birth Weight
Babies born to smokers weigh an average of 178 grams (about 6 ounces) less than babies born to nonsmokers. That may not sound dramatic, but it pushes more infants below the critical threshold of 2,500 grams (5.5 pounds), the clinical cutoff for low birth weight. Among smokers, 9.9% of babies are born at low birth weight compared to 5.7% among nonsmokers. Low birth weight babies face higher rates of breathing problems, infection, and developmental delays in infancy.
Brain Development Changes
Nicotine interferes with fetal brain development starting remarkably early. Receptors that nicotine binds to are already present in the fetal brain and spinal cord by four to five weeks of gestation, a point when many women don’t yet know they’re pregnant. These receptors play a key role in guiding how nerve cells find their targets, form connections, and survive. When nicotine floods these receptors, it disrupts that process.
The effects are measurable years later. A study published in Frontiers in Human Neuroscience found that adolescents who had been exposed to smoking before birth had smaller gray matter volumes in brain regions involved in memory and sensory processing. They also showed reduced mature nerve cell content in the prefrontal cortex, the area responsible for planning, decision-making, and impulse control. These aren’t subtle biochemical differences. They’re structural changes visible on brain imaging more than a decade after birth.
Birth Defects
Smoking during the first trimester increases the odds of a baby being born with a cleft lip (with or without cleft palate) by 1.9 times and cleft palate alone by 2.3 times. The risk also scales with how much a mother smokes, meaning heavier smoking carries a higher likelihood of these defects. Even passive smoke exposure during pregnancy raises the risk of orofacial clefts, which require surgical repair and can affect feeding, speech, and dental development.
SIDS Risk After Birth
The danger doesn’t end at delivery. Infants whose mothers smoked beyond the first trimester face a nearly fivefold increase in the risk of sudden infant death syndrome (SIDS) compared to unexposed infants. When prenatal smoking is combined with drinking alcohol during pregnancy, that risk climbs even higher, to nearly 12 times the baseline. Prenatal smoke exposure is believed to affect the parts of the developing brainstem that regulate breathing and arousal during sleep, making it harder for an infant to wake up or adjust when oxygen levels drop.
Secondhand Smoke Matters Too
You don’t have to smoke yourself to put a pregnancy at risk. Secondhand smoke exposure during pregnancy causes lower birth weight and may trigger preterm delivery. If you live with a smoker or work in an environment where smoking is common, the fetus is still absorbing harmful chemicals through your bloodstream. Reducing exposure means more than just not lighting up yourself.
Vaping Is Not a Safe Alternative
Switching to e-cigarettes during pregnancy does not eliminate the risk. Nicotine in any form damages the developing fetal brain and lungs, and e-cigarettes deliver nicotine directly. Some of the flavoring chemicals used in vaping products may pose additional harm, though research on their specific effects during pregnancy is still limited. The FDA has not approved e-cigarettes as a smoking cessation tool, and no major medical organization considers them safe during pregnancy.
Benefits of Quitting, Even Late in Pregnancy
The single most encouraging finding in the research is that quitting at any point during pregnancy improves outcomes. The greatest benefit comes from stopping before 15 weeks of gestation, which covers most of the first trimester and the early second trimester. Quitting by that point gives the fetus the best chance of catching up on growth and reduces the elevated risks of placental problems, low birth weight, and preterm birth.
But quitting later still helps. Every week without cigarette smoke means more oxygen reaching the baby, better blood flow through the placenta, and less nicotine interfering with brain development. Among women who smoked in the three months before becoming pregnant, 56.1% managed to quit during pregnancy. That number suggests quitting is difficult but far from impossible, and the biological payoff for the baby is immediate once smoking stops.