According to a 2006 study published in "National Vital Statistics Reports," at least 10 percent of women smoke while pregnant. Exposure to smoke prenatally increases the risk for several adverse outcomes for the fetus, including miscarriage and fetal death, low birthweight and preterm delivery and possibly birth defects. The more a woman smokes, the higher the risk is for these outcomes. Fortunately, decreasing or ceasing smoking during the second half of pregnancy largely reduces the risks.
Miscarriage and Infant Death
Smoking during pregnancy is one of the leading preventable causes of miscarriage and later fetal death. When the mother smokes, nicotine, carbon monoxide and other toxins enter the fetal bloodstream. Smoking also constricts blood vessels in the placenta, decreasing blood flow to the fetus. The combination of toxins and insufficient nutrients and oxygen reaching the fetus can impact fetal growth, even causing fetal demise. Babies exposed to smoke prenatally who survive pregnancy remain at increased risk for death even after birth. Results of another "National Vital Statistics Reports" study from 2006 quantified the risk for babies born to smokers; in that study, infants of smokers had a mortality rate 71 percent higher than infants of nonsmokers (11.25 percent versus 6.65 percent).
Growth Restriction, Low Birthweight and Preterm Birth
Smoking-induced blood vessel constriction can also cause growth restriction in the fetus. This can be detected during pregnancy when ultrasound measurements note that the baby's bone lengths, abdominal circumference and head circumference lag behind expected measurements. Babies with growth restriction are usually underweight at birth. Smoking during pregnancy also increases the risk of a baby being born preterm (before 37 weeks gestation). Low birthweight and preterm birth are major risk factors for complications later in the baby's life, including learning disabilities, cerebral palsy, behavioral problems and psychiatric disorders. Because most of the risk for low birthweight and preterm birth is during the second half of the pregnancy, women who stop smoking during pregnancy can greatly reduce their risk for these outcomes.
Possible Birth Defects
Exposure to smoke in the womb may also increase the fetus' risk for birth defects. Studies are inconsistent in determining this risk, in part because women who smoke during pregnancy may also drink alcohol or use other drugs, and it can be difficult to clarify how much of the risk is from smoking alone. The time of highest risk for birth defects is during the first trimester of pregnancy, when the fetal organs are forming. Many different defects including limb reduction defects (where an arm or leg fails to develop completely), heart defects, clubfoot and cleft lip with or without cleft palate have been suggested as being associated with smoking. A 2008 report from the U.S. National Birth Defects Study concluded that smoking was associated with a statistically insignificant "moderately increased" risk (1.5 times higher than the background risk) for birth defects.
References
- "National Vital Statistics Reports"; Births: Final Data for 2004; JA Martin et al; September 2006
- "National Vital Statistics Reports"; Infant Mortality Statistics from the 2003 Period Linked Birth/Infant Death Data Set; Mathews et al; May 2006
- "Birth Defects Research"; Craniosynostosis and Maternal Smoking; SL Carmichael et al; Feb. 2008
