Yes, secondhand smoke affects pregnancy in measurable ways. Breathing in someone else’s tobacco smoke exposes you and your developing baby to nicotine, carbon monoxide, and dozens of other toxic chemicals that cross the placenta and reach the fetus directly. The risks range from lower birth weight and preterm delivery to changes in fetal brain and lung development that can persist after birth.
How Secondhand Smoke Reaches Your Baby
Nicotine crosses the placenta easily, and concentrations in fetal blood and amniotic fluid can actually be higher than in the mother’s own bloodstream. Carbon monoxide from tobacco smoke binds to hemoglobin more aggressively than oxygen does, reducing the amount of oxygen delivered to the placenta and, in turn, to the baby. This combination of chemical exposure and oxygen restriction is what drives the specific pregnancy complications linked to passive smoke.
The American College of Obstetricians and Gynecologists (ACOG) states plainly that nicotine intake in any form has “considerable health risks with known adverse effects on fetal brain and lung tissue.” There is no established safe threshold for exposure during pregnancy.
Lower Birth Weight
One of the most consistent findings is that babies born to mothers exposed to secondhand smoke weigh less. After controlling for other factors like maternal age, diet, and income, one study found that secondhand smoke exposure reduced average birth weight by about 206 grams, roughly half a pound. That may not sound like much, but in a newborn it can be the difference between a healthy weight and one that puts the infant at higher risk for feeding difficulties, temperature regulation problems, and longer hospital stays.
Preterm Birth and Newborn Complications
Women exposed to secondhand smoke during pregnancy are roughly 2 to 3 times more likely to deliver preterm compared to unexposed, nonsmoking women. That risk is nearly identical to the preterm birth risk seen in women who smoke themselves. Babies born to secondhand smoke-exposed mothers are also about 2.4 times more likely to have immediate newborn complications.
Miscarriage Risk
The link between secondhand smoke and miscarriage is suggestive but not definitive. A meta-analysis of 17 studies found an 11% increase in miscarriage risk among women exposed to passive smoke, though the result did not reach statistical significance. One study that specifically looked at women’s history of miscarriage found a 21% increased risk among those exposed to secondhand smoke. The overall picture points toward a modest increase in risk, but researchers have not been able to confirm it as firmly as they have for other complications.
Effects on Fetal Lung Development
Secondhand smoke interferes with how fetal lungs grow and mature. One study found measurable reductions in fetal lung growth by 33 weeks of gestation. Nicotine changes the behavior of cells lining the airways, making them more reactive and increasing the likelihood of breathing problems after birth. Babies exposed to nicotine in the womb are predisposed to bronchial hyper-responsiveness, which means their airways overreact to irritants. This is one reason why children of smoke-exposed pregnancies have higher rates of asthma and wheezing in their first years of life.
Effects on Fetal Brain Development
The brain is particularly vulnerable to nicotine during fetal development. Nicotine disrupts the normal wiring of nerve cells by interfering with key chemical messengers, including serotonin and dopamine. Animal studies show that prenatal nicotine exposure leads to deficits in both of these neurotransmitters, which play central roles in mood regulation, attention, and learning. The result is altered nerve cell connections: neurons end up paired with the wrong target cells, leading to impaired communication across the brain.
Nicotine also reduces total cell numbers in specific brain regions. The area most severely affected controls movement coordination and habit learning. Prenatal smoke exposure has been linked to reduced mass in the frontal lobe (which governs decision-making and impulse control) and the cerebellum (which coordinates movement). These structural changes are associated with impairments in learning, memory, hearing, and behavior that may show up as the child grows.
SIDS Risk After Birth
Smoke exposure during pregnancy is one of the strongest modifiable risk factors for sudden infant death syndrome (SIDS). After adjusting for other factors like sleep position and socioeconomic status, prenatal smoke exposure increases the odds of SIDS by about 2.6 times. Among infants born to women who smoked during pregnancy, an estimated 61% of SIDS cases were directly attributable to the smoking. While these numbers come from studies of active smoking, the biological mechanisms are the same: nicotine alters the brain circuits that regulate breathing and arousal during sleep.
When the Smoker Is Your Partner
A partner who smokes is the single biggest predictor of a pregnant woman’s secondhand smoke exposure. In a study of nonsmoking pregnant women, 94% of those with a smoking partner had detectable nicotine byproducts in their urine, compared to about 60% of those with a nonsmoking partner (that 60% likely reflects workplace or public exposure). Having a smoking partner made a woman nearly 8 times more likely to have elevated cotinine, the chemical your body produces when it processes nicotine.
Higher cotinine levels were directly tied to lower birth weights. For every increase in the mother’s cotinine level, birth weight dropped, with one analysis showing a reduction of roughly 281 grams. Interestingly, this effect was more pronounced in male newborns, where the reduction reached about 470 grams, just over a pound. Research from China has also found that paternal smoking is independently associated with preterm birth, and some evidence links it to a higher risk of childhood leukemia in offspring.
Thirdhand Smoke Matters Too
Even when no one is actively smoking nearby, tobacco residue settles on clothing, furniture, car seats, and walls. This is sometimes called thirdhand smoke. These residues continue to release toxic chemicals over time, and they can be inhaled or absorbed through the skin. Studies have found that thirdhand smoke chemicals cause the same types of lung damage seen with direct secondhand exposure, including disrupted signaling in developing lung tissue. If you live in a home where someone smokes, even if they step outside, some exposure likely remains on surfaces and fabrics indoors.
Reducing Your Exposure
The most effective step is eliminating smoke from your home and car entirely. Asking a partner or household member to smoke outside helps but does not eliminate exposure completely, since residues cling to hair, skin, and clothing. If your partner smokes, their quitting during the pregnancy is the single most impactful change for your baby’s health. ACOG recommends that healthcare providers screen for tobacco and nicotine exposure at every prenatal visit and connect patients with cessation resources.
At work or in public spaces, staying upwind of smokers and avoiding enclosed areas where people smoke reduces the concentration of chemicals you breathe. If you live in a multi-unit building where smoke drifts between apartments, sealing gaps around doors, windows, and shared walls can help, though it will not block all exposure. The goal is to get your exposure as close to zero as possible, because no safe level has been identified for a developing pregnancy.