Researchers said Sunday they had the first hard evidence that pregnant smokers can pass cancer-causing substances to their babies.
They said they found evidence of nicotine in the urine of newborn babies, which they said proves that tobacco products cross the placenta into a baby in the womb.
Stephen Hecht of the University of Minnesota and colleagues said they found by-products of NNK, a chemical derived from nicotine, in the urine of newborn babies.
They tested 48 samples sent by collaborators in Germany. Of those, 31 of the babies were born to mothers who smoked during pregnancy. And of those 31 samples, 22 contained the NNK.
Nicotine is not considered to be a cancer-causing agent, but it indicates that other chemicals from cigarettes are in the body.
"The results demonstrate that uptake of NNK . . . begins before birth," Hecht told a meeting of the American Chemical Society in Boston.
"To identify the carcinogens in the urine of a newborn is a major documentation of the potential role and the transmission of those compounds," he said.
He said the amount of NNK found in the newborns' urine was about 10 percent of that found in the urine of adult smokers.
Although smoking is known to increase the risk of having a low-birthweight baby and also increases the risk of losing a baby to Sudden Infant Death Syndrome (or crib death), up to 61 percent of women who smoke do not quit during pregnancy.
Last year, Hecht reported that NNK had been found in the urine of nonsmoking adults who were exposed to cigarette smoke at work.