Smoking during pregnancy increases the risk of a wide range of birth defects, including skull defects, missing or deformed limbs, clubfoot, cleft palate, protrusion of the gastrointestinal system through the skin and heart problems, according to the first major study to examine the incidence of such risks.
Smoking during pregnancy is already well known for increasing the risk of miscarriage, low birth weight and premature birth, said Dr. Allan Hackshaw of the University College London Cancer Institute, who led the study, published in the journal Human Reproduction Update. But “very few public health educational policies mention birth defects when referring to smoking and those that do are not specific — this is largely because of past uncertainty over which ones are directly linked,” he said in a statement. “Now there is evidence,” said Dr. Michael Katz, senior vice president for research and global programs of the March of Dimes.
Precise numbers for birth defects are somewhat difficult to come by. A University of Kansas Medical Center website says 3% to 5% of U.S. infants have a congenital anomaly at birth. A study by the Environmental Protection Agency conducted during the early 2000s found that the incidence among white infants was 1.0%, while the rate among black infants was 1.3%. The most common birth defect in that study was heart defects, which account for about one in every eight congenital problems. Each year in the United States, at least 120,000 infants are born with major structural abnormalities, according to the March of Dimes.
Despite the well-known problems associated with tobacco use, a surprising number of women still smoke during pregnancy. In the United States, the March of Dimes says, about 20% of women do so, with the highest rate among pregnant teenagers and the lowest rate among women over 35. Poorly educated and economically disadvantaged women are more likely to smoke than those with higher educations and better jobs. In Britain, Hackshaw said, 17% of pregnant women smoked, including a “staggering” 45% of those under age 20.
Hackshaw and his colleagues surveyed the medical literature for the last 50 years looking for studies that examined smoking and birth defects. They ultimately isolated 172 reports that, collectively, found 173,687 infants born with birth defects and 11,674,332 healthy controls. Analyzing the data, they found:
-- A 33% increased risk of skull defects.
-- A 28% increased risk of being born with a clubfoot.
-- A 28% increased risk of cleft lip or cleft palate.
-- A 27% increased risk of gastrointestinal defects.
-- A 26% increased risk of missing or deformed limbs.
-- A 10% increased risk of heart defects.
-- A 50% increased risk of gastroschisis, in which parts of the stomach or gut protrude through the skin of the abdomen.
The study did not examine how smoking creates these problems. But cigarette smoke contains as many as 4,000 different chemicals, many of which could be teratogenic, meaning they could interfere with the development of a fetus. Moreover, smoking and the carbon monoxide in smoke both constrict blood vessels, impairing circulation and potentially limiting the supply of oxygen to the fetus.
“Now we have this evidence, advice should be more explicit about the kinds of serious defects ... that babies of mothers who smoke during pregnancy can suffer from,” Hackshaw said. “The message from this research is that women should quit smoking before becoming pregnant, or very early on, to reduce the chance of having a baby with a serious and lifelong physical defect.”