Congenital anomalies and smoking

DEFINITION: It was previously estimated that between 10 and 25 percent of women smoked during their pregnancies. Incidence rates were especially high among young, single women with low incomes. By the mid-2020s, incidences of smoking in pregnant women had dropped dramatically, with 4.6 percent of women reporting smoking during pregnancy. Prenatal exposure to nicotine results in many negative effects on the developing fetus, including physical and cognitive problems evident at birth and throughout the lifespan.

Mechanism of Action

The pharmacologically active ingredient in cigarettes is nicotine. When smoked during pregnancy, nicotine passes into the bloodstream of the fetus. After nicotine crosses the placenta, the level of nicotine in the fetus exceeds that of the level in the pregnant person. Nicotine can also constrict blood vessels in the umbilical cord, which decreases the amount of oxygen available to the fetus. The effect of nicotine on fetal development appears to be dose-dependent: the more cigarettes smoked during a pregnancy, the greater the risk to the fetus.

Prenatal Exposure to Nicotine: Physical Effects

Fetuses exposed to nicotine are less likely to survive pregnancy than those not exposed. Smoking increases the likelihood of ectopic pregnancy, spontaneous abortion, and placental abruption (in which the placenta detaches from the uterine wall), which can lead to perinatal mortality (death during the end of pregnancy or within the first month after birth).

Fetuses exposed to nicotine prenatally are at a greater risk of being born prematurely, having a low birth weight (an average weight reduction of 200 grams), showing heightened tremors and startles, and having respiratory problems. Prenatal exposure to nicotine also increases the risk of stillbirth and infant mortality. The rate of sudden infant death syndrome is substantially higher among exposed infants than it is among infants who were not exposed to nicotine before birth.

Research on the relationship between maternal smoking and the risk of congenital abnormalities has produced mixed results. While some researchers suggest a link between maternal smoking and, for example, cleft palate, this result is not found in all studies. There is evidence to support an increased risk of cardiovascular anomalies and neural tube defects in infants who were exposed to nicotine before birth. Data from the National Birth Defects Prevention Study also reveals an increase in craniosynostosis (in which the bones in the skull of a fetus close too early) in fetuses carried by women who smoked more than fifteen cigarettes per day while pregnant.

The problems associated with prenatal exposure to nicotine continue beyond infancy. Children who had been prenatally exposed to nicotine continue to be at risk for poor respiratory function during childhood and are more likely to be diagnosed with asthma and wheezing than are children who were not exposed. Children born to women who smoked during their pregnancies are also at an increased risk of developing several types of childhood cancer.

While most of the research in this field has focused on pregnant women who smoke, some work looking at secondhand smoke suggests that exposure to secondhand smoke may increase fetal risk at a rate that is similar to that seen in women who smoke less than one-half a pack per day while pregnant.

Prenatal Exposure to Nicotine: Cognitive Effects

When women smoke during pregnancy, their children have (when tested at three and four years of age), on average, poorer language skills and lower cognitive functioning than children not exposed. Many studies report that children prenatally exposed to nicotine are at an increased risk for intellectual impairment. Furthermore, controlled studies have documented a 50 percent increase in idiopathic intellectual disability (in which a low intelligence quotient is not associated with a chromosomal defect).

Children whose mothers smoked during pregnancy are also more likely to be diagnosed with attention-deficit hyperactivity disorder than their peers who were not exposed. In addition to having academic difficulties, these children are also more likely to have disruptive behavior disorders or be diagnosed with a psychiatric disorder during childhood, especially among those exposed to heavy smoking.

Paternal Smoking

Although most studies focus on maternal smoking, some researchers have explored the effect of paternal smoking on fetuses and found evidence that men who smoked for years before conceiving a child had children who were at increased risk for developing brain tumors and cancer before the age of five. Other researchers have reported an increased risk of anencephalus (the absence of a large part of the brain and skull) and of spina bifida (in which some vertebrae are not fully formed, preventing their closure around the spinal cord) in the offspring of fathers who smoked.

As the dangers of smoking and pregnancy have become increasingly apparent, a sharp decline in expectant mothers reporting nicotine and tobacco use has occurred. Between 2016 and 2021, the percentage of women who reported smoking during pregnancy declined by 36 percent in the United States. The global prevalence of smoking during pregnancy remains high in some areas, such as Europe, although England has seen declines similar to the US. These declines demonstrate progress, partly due to public health education campaigns and other tools for protecting maternal and infant health, including medications. Some research indicates that several quit-smoking medicines are safer to take during pregnancy than continuing to smoke.

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