MONDAY, Feb. 2, 2009 (HealthDay News) -- A mechanism linking smoking during pregnancy to low birth weight has been identified by Danish researchers.
They found that smoking during pregnancy lowers production of an enzyme called endothelial nitric oxide synthase (eNOS), which regulates blood vessel dilation. Reduced production of eNOS causes narrowing of blood vessels and less blood flow to the fetus, resulting in lower birth weight, shorter length and smaller head circumference.
But the researchers also found that if women quit smoking early in pregnancy, eNOS levels return to normal, and infants are born at normal birth weight. The findings were published in the Feb. 2 online issue of Circulation.
"This study in one of the first to show a biochemical measurement of what is going on to cause the lower birth weight," said study author Malene Rohr Andersen, project manager of the Department of Clinical Biochemistry at Gentofte University Hospital.
"We have known for 50 years that infants born to mothers who smoke during pregnancy have lower birth weights. But this study provides a possible explanation of why there is restricted blood flow to the fetus. Reduced production of the vasodilator nitric oxide may lower the blood flow to the fetus," Andersen said in an American Heart Association news release.
The researchers looked at 266 healthy pregnancies of 182 nonsmokers, 43 smokers, and 41 women who stopped smoking at about six weeks into their pregnancy. The more cigarettes smoked per day, the lower the levels of eNOS concentration and activity.
The average difference in the newborn weight of infants born to nonsmokers and smokers was about 350 grams. Differences in eNOS activity accounted for about 25 percent of the birth weight reduction among infants born to smokers.
"Women who smoke should stop smoking if they plan to become pregnant, or at the very least should stop smoking as soon as they find out they are pregnant," Andersen said. "Women should also not start smoking again once they have delivered. Some women stop smoking during pregnancy, only to start up again after the baby is delivered, and the child is then exposed to environmental smoke, which also affects vascular function."
The American Lung Association has more about smoking and pregnancy.