What Does Smoking Do to a Fetus During Pregnancy?

Smoking during pregnancy disrupts nearly every aspect of fetal development, from oxygen delivery and organ growth to brain wiring and immune programming. The damage starts with the hundreds of toxic chemicals in cigarette smoke crossing the placenta and entering the fetal bloodstream, where they interfere with how the baby grows, breathes, and builds its organs. The effects are wide-ranging and, in some cases, persist well beyond birth.

How Smoking Cuts Off Oxygen and Nutrients

The most immediate harm begins with two chemicals: nicotine and carbon monoxide. Nicotine triggers the release of stress hormones into the mother’s bloodstream, which causes blood vessels to constrict and reduces blood flow through the placenta. Less blood flow means less oxygen and fewer nutrients reaching the fetus. Carbon monoxide compounds the problem by binding tightly to fetal hemoglobin, the molecule that carries oxygen in the baby’s blood. Because carbon monoxide latches on much more aggressively than oxygen does, the fetus ends up in a state of chronic low-grade oxygen deprivation.¹

The fetal heart responds almost immediately when a pregnant person smokes. Studies using Doppler ultrasound have documented a rise in fetal heart rate within minutes of the mother lighting a cigarette, along with measurable changes in how the fetal aorta pulses.² These aren’t subtle, long-term trends. They happen in real time, each time a cigarette is smoked, as the baby’s cardiovascular system scrambles to compensate for reduced oxygen.³

Smaller Babies and Restricted Growth

One of the best-documented consequences of smoking during pregnancy is that babies are born smaller. Research consistently links smoking to fetal growth restriction, where the baby doesn’t reach its expected size, and to low birth weight, typically defined as under about 5.5 pounds. In one study, women who smoked in the first trimester had roughly four and a half times the odds of delivering a baby below the 10th percentile for birth weight, and more than six times the odds of having a low-birth-weight infant, compared to women who never smoked.⁴ The risk climbed even higher when the mother was also carrying excess weight, suggesting the two exposures amplify each other.

Both active and passive smoking during pregnancy have been tied to these growth problems, and the relationship between exposure and outcomes follows a dose pattern: the more smoke exposure, the worse the growth restriction tends to be.⁵ Low birth weight isn’t just a number on a chart. It raises the odds of feeding difficulties, infections, and extended hospital stays in the newborn period, and it has been linked to chronic health problems later in life.

Placental Damage and Preterm Delivery

The placenta itself takes a beating from cigarette smoke. In pregnancies complicated by placental abruption, where the placenta separates from the uterine wall, researchers found that smokers had dramatically higher rates of blood clots within the placental tissue. About one in five smokers showed these clots compared to roughly one in thirty nonsmokers.⁶ A damaged placenta can’t do its job well, and when the damage becomes severe enough, it can trigger early delivery.

Smoking also raises the risk of preterm premature rupture of membranes, one of the most common pathways to preterm birth. Women who smoked at any point during pregnancy had about 60% higher odds of their membranes rupturing before 37 weeks, compared with nonsmokers.⁷ Smoking increases the risk of very preterm birth through multiple mechanisms, including spontaneous preterm labor and late-pregnancy bleeding.⁸ Babies born very early face a cascade of their own medical challenges, from immature lungs to fragile blood vessels in the brain.

A Developing Brain Under Chemical Assault

The fetal brain is exquisitely sensitive to chemical disruption, and cigarette smoke delivers thousands of compounds directly into the environment where that brain is being built. Reviews of the evidence show that prenatal smoking exposure restricts both overall body growth and head growth, and that children exposed in utero show measurable alterations in brain structure and function.⁹

The mechanism behind these brain effects is partly about nicotine and partly about oxygen. Nicotine crosses the placenta easily and interferes with the receptors in the fetal nervous system that help guide how neurons connect and communicate. It also disrupts the normal functioning of neurotransmitters, the chemical messengers the brain relies on. Meanwhile, carbon monoxide reduces blood flow and oxygen delivery to the developing central nervous system. Together, these disruptions may help explain why children born to mothers who smoked during pregnancy show higher rates of cognitive delays and language problems.¹⁰

Behavioral and psychiatric outcomes are affected too. A large cohort study found that children whose parents smoked both before and during pregnancy were about twice as likely to be diagnosed with ADHD compared with children of nonsmoking parents.¹¹ Untangling cause from correlation in studies like these is always tricky, because smoking during pregnancy correlates with other factors that could independently affect child behavior. But the biological plausibility is strong, given nicotine’s direct effects on fetal brain receptors.

Lungs That Start Behind

Maternal smoking during pregnancy has been called the largest preventable cause of abnormal lung development in utero. The damage shows up as decreased airflow, reduced lung compliance (meaning the lungs don’t stretch and fill as easily as they should), more hospitalizations for respiratory infections, and higher rates of childhood wheezing and asthma.¹²

Animal studies help explain why. In mice, maternal smoking during pregnancy turned down the activity of genes involved in lung development in newborn offspring and led to airway remodeling, structural changes in the airways, in adult offspring.¹³ In other words, the damage isn’t just temporary. The lungs may be built differently from the start, and those structural problems can persist into adulthood.

Heart Defects and Congenital Malformations

A meta-analysis pooling data from studies involving more than 19,000 cases found that maternal smoking at any point during pregnancy was associated with about a 10% increase in the odds of congenital heart defects. The link was somewhat stronger for a specific category of defects called conotruncal defects, which involve the outflow tracts of the heart, where the odds were raised by roughly 14%.¹⁴ A 10% increase may sound modest, but congenital heart defects are already among the most common birth defects. Applied to millions of births each year globally, even a small percentage increase translates to thousands of additional affected infants.

Changes Written Into the DNA

Some of smoking’s effects on the fetus aren’t visible on an ultrasound or at birth. They’re written into how the baby’s genes are regulated, a field known as epigenetics. Smoking during pregnancy alters DNA methylation, a chemical process that helps control which genes are turned on or off, without changing the underlying genetic code itself. A large consortium meta-analysis identified over 6,000 sites across the genome where methylation was significantly different in newborns whose mothers had smoked, including nearly 3,000 sites that had never previously been connected to smoking.¹⁵

These aren’t just lab curiosities. Methylation changes linked to maternal smoking have been found in the placenta itself, clustered around genes involved in growth factor signaling, inflammation, and the body’s response to environmental stressors.¹⁶ And the marks don’t necessarily disappear at birth. Growing evidence from longitudinal studies shows that some of these methylation changes persist through childhood, raising the question of whether they contribute to the long-term health risks seen in smoke-exposed children.¹⁷

SIDS and the Brainstem Connection

Prenatal smoking exposure is one of the strongest modifiable risk factors for sudden infant death syndrome. Estimates suggest that in-utero nicotine exposure raises SIDS risk by two- to fivefold.¹⁸ The connection appears to run through the brainstem, where nicotine disrupts the development of circuits that control breathing, heart rate, and arousal from sleep. In animal models, chronic nicotine exposure during pregnancy shifted the fetal autonomic nervous system toward parasympathetic dominance, with a roughly 55% increase in one measure of heart rate variability, suggesting the normal balance of the infant’s automatic control systems was thrown off.

Research on human SIDS cases has found that infants exposed to cigarette smoke in utero showed abnormal nicotinic receptor development in brainstem regions tied to arousal and breathing control. Specifically, the expected adaptive changes in receptor density that should occur with chronic nicotine exposure didn’t happen in these infants, suggesting the brainstem circuits responsible for waking a baby up when breathing falters may be fundamentally compromised.¹⁹

Immune Programming and Allergy Risk

The fetal immune system is being trained during pregnancy, and smoking appears to interfere with that training. Research has found that prenatal smoke exposure weakens innate immune responses, specifically the toll-like receptor pathways that recognize and respond to microbial threats. These pathways play a central role in teaching the immune system to distinguish real threats from harmless substances like pollen or food proteins. When they don’t develop properly, the child may be predisposed to allergic disease.²⁰ Separate studies have confirmed that maternal smoking alters neonatal immune cell responses, changing the pattern of signaling molecules that newborns produce.²¹

Long-Term Metabolic Consequences

The fetus exposed to smoke appears to undergo a kind of metabolic reprogramming. Children and adults who were exposed to cigarette smoke during fetal development show higher rates of obesity and cardiovascular disease.²² Studies specifically tracking childhood weight have found a dose-response relationship: the more the mother smoked, the higher the child’s risk of becoming obese.²³ The combination of restricted fetal growth followed by rapid catch-up weight gain after birth is thought to be one mechanism behind this pattern, though the epigenetic changes described earlier likely contribute as well.

Secondhand Smoke Is Dangerous Too

Pregnant people who don’t smoke themselves but live or work around smokers face real risks. A meta-analysis estimated that secondhand smoke exposure during pregnancy increased the likelihood of stillbirth by about 23% and the likelihood of congenital malformation by about 13%.²⁴ In a study comparing exposed and unexposed groups, preterm delivery occurred in roughly a quarter of secondhand-smoke-exposed pregnancies versus about one in ten of those without exposure.²⁵

The dose matters. When researchers measured nicotine in maternal hair as an objective marker of passive smoke exposure, they found that concentrations above a certain threshold were associated with a sixfold increase in the risk of premature delivery. Living with a partner who smoked more than a pack a day quadrupled the risk.²⁶ These findings underscore that smoke exposure is not something only the person holding the cigarette needs to worry about.

E-Cigarettes Are Not a Safe Alternative

Some pregnant people switch to vaping under the assumption that e-cigarettes are less harmful. The evidence so far does not support the idea that they’re safe for fetal development. The nicotine in e-cigarettes can impair lung development at both structural and molecular levels, and the solvents and flavorings used in vape liquids trigger inflammation and oxidative stress that may compromise newborn lung function. Research suggests prenatal e-cigarette exposure may raise the risk of asthma and chronic lung disease later in life.²⁷

Perhaps more surprising, a recent study found that even nicotine-free flavored e-cigarettes caused abnormal embryo development and elevated blood cell accumulation at implant sites, a pattern that may contribute to early pregnancy loss.²⁸ The flavorings alone, independent of nicotine, appear to be biologically active enough to cause harm. This is an evolving area of research, but the early signals are not reassuring.

When Quitting Makes a Difference

The encouraging news is that quitting smoking during pregnancy genuinely reduces the risk. The earlier a person quits, the more protection the fetus gets. Quitting before the 15th week of pregnancy appears to offer the greatest benefit.²⁹ The risk of fetal growth restriction specifically decreases the earlier cessation occurs, though smoking for any duration during pregnancy still carries some elevated risk compared to never smoking.³⁰

Nicotine replacement therapy, like patches or gum, occupies an awkward middle ground. The available evidence suggests it’s likely safer than continued smoking, but nicotine itself is not harmless to the fetus. Different NRT products carry different regulatory warnings, with patches and inhalers classified more strictly than gum or lozenges.³¹ The practical takeaway is that behavioral support to quit without any nicotine product is ideal, but if the choice is between NRT and continuing to smoke cigarettes with their full cocktail of toxins, NRT is the lesser harm.

Heavy Metal Exposure Through the Placenta

Beyond nicotine and carbon monoxide, cigarette smoke delivers heavy metals to the fetus. Cadmium, a carcinogenic metal concentrated in tobacco leaves, accumulates in the placenta and maternal blood of smokers. One study found that smoking increased cadmium levels in the placenta by about 32% and in maternal blood by about 59%, with higher levels also measured in cord blood.³² Cadmium is known to interfere with zinc and calcium metabolism, both of which are critical for fetal growth and bone development. The placenta acts as a partial filter, but it can’t keep everything out, and with chronic smoking, these metals build up in the very tissue that’s supposed to protect the baby.

Effects That Reach the Next Generation

Some of the most striking recent research suggests that smoking during pregnancy doesn’t just affect the immediate child. It may affect grandchildren. The mechanism involves damage to the fetal germ cells, the cells in the developing baby that will eventually become its own eggs or sperm. In animal studies, male offspring of mothers exposed to cigarette smoke during pregnancy had sperm counts reduced by 40-60%, and their sons, who were never directly exposed, still showed sperm count reductions of 23-40%.³³

Detailed examination of male offspring exposed in utero revealed increased germ cell death, depleted stem cell populations in the testes, and structural disorganization of the tissue where sperm are produced. These changes persisted into adulthood even though the exposure ended at birth. The affected males took longer to achieve pregnancies and sired slightly smaller litters.³⁴ While these are animal studies and human confirmation is still limited, the findings align with the epigenetic data showing that smoking leaves lasting molecular marks on fetal tissue.

How Doctors Can Detect Fetal Exposure

When there’s a question about whether a fetus or newborn was exposed to tobacco, clinicians have several ways to check. Nicotine and its breakdown products, particularly cotinine and hydroxycotinine, can be measured in both meconium (the baby’s first stool) and umbilical cord tissue. The two specimens show slightly different chemical patterns: cotinine is more prominent in cord tissue, while nicotine itself shows up at higher levels in meconium. Either specimen can reliably detect third-trimester exposure.³⁵

For earlier stages of pregnancy, other biological samples come into play. Amniotic fluid and fetal hair can provide evidence of exposure at different gestational windows, while cord blood and neonatal urine capture exposure closer to delivery.³⁶ These biomarkers are useful in research settings to objectively verify smoking status, since self-reported data tends to underestimate actual exposure. In clinical practice, they occasionally help identify infants who may need closer monitoring for smoke-related complications that could emerge in the first weeks and months of life.