Smoking during pregnancy restricts oxygen to the developing baby, increases the risk of preterm birth by up to 45%, and roughly doubles the chance of a dangerously low birth weight. Every cigarette introduces nicotine and carbon monoxide into the bloodstream, and both cross the placenta freely. The effects reach nearly every system in the baby’s body, from the lungs and heart to the brain.
How Smoking Starves the Baby of Oxygen
Two components of cigarette smoke do the most direct damage to a developing fetus: nicotine and carbon monoxide. They work through different pathways, but both reduce how much oxygen reaches your baby.
Nicotine constricts blood vessels, including the uterine artery that supplies the placenta and potentially the umbilical artery that feeds the baby directly. Narrower vessels mean less blood flow, and less blood flow means less oxygen and fewer nutrients making the trip. Carbon monoxide, meanwhile, binds to hemoglobin in the blood and forms a compound that physically blocks oxygen from being released into fetal tissues. So even the blood that does reach the baby is carrying less usable oxygen than it should. Together, these two effects create a persistent state of oxygen deprivation that slows fetal growth throughout pregnancy.
Lower Birth Weight
One of the most well-documented consequences of smoking during pregnancy is a smaller baby at birth. On average, babies born to mothers who smoke weigh about 330 to 350 grams (roughly three-quarters of a pound) less than babies born to nonsmokers. In heavy smokers, that gap can widen dramatically, with some studies recording an average reduction of over 750 grams.
Overall, smoking during pregnancy doubles the risk of low birth weight, typically defined as under 5.5 pounds. Low birth weight isn’t just a number on a scale. These babies face higher rates of infection, breathing problems, difficulty regulating body temperature, and longer stays in the neonatal intensive care unit. The effects can extend well beyond the newborn period, with low birth weight linked to developmental delays and chronic health problems later in life.
Preterm Birth
Smoking into the second trimester raises the risk of delivering prematurely by about 45% compared to nonsmokers. Even light smoking, just one or two cigarettes a day continued into the first trimester, is associated with a 13% increased risk of preterm birth. The earlier a baby arrives, the less time its organs have had to mature, which is why prematurity remains one of the leading causes of infant complications worldwide.
There is meaningful good news here, though. Women who quit smoking in the three months before becoming pregnant, even those who previously smoked 20 or more cigarettes a day, show essentially the same preterm birth risk as women who never smoked. Quitting early in pregnancy still helps significantly, and quitting before the 15th week of pregnancy captures most of the protective benefit.
Placental Complications
The placenta is the baby’s lifeline, and smoking directly damages it. Placental abruption, where the placenta separates from the uterine wall before delivery, occurs 1.4 to 2.5 times more often in smokers than in nonsmokers. A severe abruption can cut off the baby’s blood supply entirely, and smoking is associated with a 2.5-fold increase in abruptions severe enough to cause fetal death.
These placental problems also raise the risk of heavy maternal bleeding, emergency delivery, and long-term complications for subsequent pregnancies. Because the damage to the placenta accumulates over the course of pregnancy, every week of continued smoking compounds the risk.
Damage to Developing Lungs
Nicotine doesn’t just affect the baby indirectly through reduced blood flow. It activates receptors that are present at high concentrations in developing lung tissue, directly altering how the lungs form. Research published in the American Journal of Respiratory Cell and Molecular Biology found that prenatal nicotine exposure causes the airways to grow longer and narrower than normal, with an increased number of small airways packed into the same overall lung volume. The walls around these airways also develop excess collagen, a stiffening protein that persists into adulthood.
The practical result is reduced airflow. Babies exposed to nicotine in the womb have lower forced expiratory flows, meaning they can’t push air out of their lungs as efficiently. This isn’t a temporary setback. The structural changes to the airways are permanent, and they set the stage for a higher risk of asthma, wheezing, and respiratory infections throughout childhood and beyond. The critical window for this damage spans roughly the second half of pregnancy through the first week after birth.
Brain Development and SIDS Risk
Nicotine interferes with the development of brain systems that control breathing, heart rate, and arousal from sleep. These are the same systems that fail during sudden infant death syndrome (SIDS), which is why prenatal smoking exposure is one of the strongest modifiable risk factors for SIDS. Most SIDS deaths occur between 2 and 4 months of age, precisely when the brain regions responsible for automatic cardiorespiratory control are still maturing.
Research suggests nicotine disrupts the serotonin signaling pathways that help a sleeping baby respond to drops in oxygen or rises in carbon dioxide. A baby with normal brainstem function will gasp, turn its head, or wake up. A baby whose serotonin system was altered by prenatal nicotine exposure may not. This connection holds whether nicotine comes from cigarettes or smokeless tobacco products, reinforcing that nicotine itself, not just the smoke, is a core part of the problem.
Birth Defects
Smoking during the first trimester, when organs are forming, is linked to specific structural birth defects. The strongest evidence connects maternal smoking to oral clefts (cleft lip and cleft palate), with researchers concluding the relationship is likely causal based on the consistency and strength of the data across multiple studies.
Certain heart defects also occur more frequently. Babies born to first-trimester smokers show a 48% increased risk of pulmonary valve abnormalities, a 71% increased risk of pulmonary artery abnormalities, and a 22% increased risk of a specific type of hole between the heart’s upper chambers. Heart defects already affect roughly 1% of all newborns, so any additional risk from a preventable exposure is significant.
E-Cigarettes Are Not a Safe Alternative
Some women switch to vaping during pregnancy, assuming it eliminates the risks. It does not. E-cigarettes still deliver nicotine, which is responsible for many of the harms described above: constricted blood vessels, altered lung development, disrupted brain maturation, and increased SIDS risk. The CDC notes that the flavorings used in e-cigarettes may carry their own risks to fetal development. Because e-cigarettes are relatively new, long-term safety data during pregnancy is limited, but the known effects of nicotine on a developing baby apply regardless of how that nicotine is delivered.
The Benefits of Quitting Early
The single most important takeaway from the research is that quitting works, and quitting earlier works better. Women who stop smoking before conception return to essentially the same risk profile as nonsmokers for preterm birth. Quitting before week 15 of pregnancy captures the largest reduction in complications related to fetal growth and placental health. Even quitting later in pregnancy still provides measurable benefits, because the baby continues gaining weight and developing lung tissue right up until delivery.
The damage from smoking is cumulative and dose-dependent. Every cigarette avoided means slightly more oxygen reaching the baby, slightly less nicotine disrupting organ development, and a meaningfully better chance of a healthy outcome.