Smoking moms pass carcinogen to infants.

Cigarettes are one craving that doctors have long said pregnant women shouldn't indulge. Now, expectant mothers have yet another reason to heed that advice. According to a new study, pregnant women who smoke transmit a potent cancer-causing substance to their babies.

In urine samples collected from newborn infants of mothers who smoked during pregnancy, researchers at the University of Minnesota Cancer Center in Minneapolis detected compounds that the body produces in breaking down nicotine-derived nitrosaminoketone (NNK). That chemical, found only in tobacco, induces a variety of cancers in rodents.

Chemist Stephen S. Hecht presented the findings this week at a Boston meeting of the American Chemical Society.

According to a study in the May 1990 American Journal of Public Health, 61 percent of women who smoke continue to do so during pregnancy. Epidemiological studies have linked smoking to low birth weight and developmental problems, Hecht says. What health effects NNK might have on a developing fetus have yet to be determined. Nevertheless, he says, "we feel this presents an unacceptable risk to the fetus. It's another reason for expectant mothers to quit smoking."

Urine samples from 48 infants were collected by doctors in Dusseldorf, Germany, and sent to the Minnesota researchers. Of the 31 infants whose mothers smoked during pregnancy, 22 had NNK metabolites called NNAL and NNAL-Gluc in their urine. In contrast, none of the 17 infants whose mothers were nonsmokers produced these substances.

The urine of the smokers' newborn also contained nicotine and one of its breakdown products. Nicotine is addictive but is not known to be carcinogenic.

The nine smokers in the study whose infants did not appear to produce the NNK metabolites smoked less than the other study participants, Hecht says. The results signal that "if [pregnant women] can't quit, they ought to smoke less."

The new findings "provide the first direct evidence that a tobacco-specific carcinogen from maternal smoking passes the placenta into the human fetus--and in a considerable amount," says Lucy M. Anderson of the National Cancer Institute in Frederick, Md.

Previous studies showed that NNK metabolites themselves don't readily cross to a fetus. While their concentration in the babies' urine was only about one-tenth that in their mothers', their presence indicates "the human fetus has a well-developed capacity to metabolize a tobacco-specific carcinogen," Anderson says.

Last year, Hecht and his colleagues reported that people exposed to environmental, or secondhand, smoke also absorb and process NNK (SN: 9/20/97, p. 188). The infants in the current research produced a urine concentration of NNK metabolites that is about three times that measured in urine of people breathing secondhand smoke.

Scientists' awareness of NNK's carcinogenic effects on fetuses comes from animal studies. The offspring of female hamsters exposed to NNK develop tumors readily. In mice, however, NNK seems less potent. "We have no idea where the human falls in this [range]," Anderson says.

Hecht worries that exposure to NNK in the womb might predispose infants to cancer later in life. The substance induces cancers by causing mutations in DNA. Because the cells of a fetus are rapidly dividing, DNA changes that occur early might propagate widely. Then again, Hecht notes, the enzymes required to induce cancer in adults may not yet be present in a developing fetus.

The method used in the Minnesota study offers a new way to explore the effects of smoking and other behaviors during pregnancy, says Anderson. "Simple, old-fashioned epidemiological studies have not shown a strong correlation between childhood cancers and maternal smoking. We have to look at it in a more sophisticated way."