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updated 1/16/2012 5:08:14 PM ET 2012-01-16T22:08:14

Any drinking during pregnancy increases the odds of fetal alcohol syndrome, but the risk to the fetus is highest if a pregnant woman drinks during the second half of her first trimester of pregnancy, a new study finds.

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For every one drink per day increase in alcohol intake during that crucial period, a woman’s baby was 25 percent more likely to have an abnormally shaped lip, 12 percent more likely to have a smaller-than-normal head and 16 percent more likely to have low birth weight — all early signs of fetal alcohol syndrome, the study showed.

"The take-home message is that there's not a low threshold level below which drinking alcohol doesn't raise the risk," of fetal alcohol syndrome, said study author Dr. Christina Chambers of the University of California, San Diego. "This supports the surgeon general's recommendation that drinking be avoided entirely."

The new findings were published in the journal Alcoholism: Clinical and Experimental Research.

Pregnancy and substances
Chambers and her colleagues recruited the 992 participants for their study from pregnant women who called a California telephone help line that answers questions on substances that could be harmful during pregnancy — including not only alcohol and illegal drug intake, but also chemical exposures and prescription drug safety.

Women who called the line between 1978 and 2005 were asked if they'd like to participate in a follow-up study. All were contacted by phone for follow-up on levels of alcohol consumption throughout pregnancy, and their infants were screened after birth with a full physical exam.

"You're dealing with an issue here in which it's really hard to get good information on both exposure and outcomes," Chambers said. "Most kids don't get diagnosed until they're in school and having learning difficulties," Chambers said.

In the study, the doctors who examined the infants did not know whether the child had been exposed to alcohol or some other substance in utero.

Higher alcohol consumption of a mother during pregnancy, the researchers found, was linked with a higher chance of a baby having physical characteristics associated with fetal alcohol syndrome, including abnormal head size and altered shape of the eyes and lips. Such symptoms suggest the presence of related neurological problems.

Binge drinking, which involves drinking more than four drinks on a single occasion, didn't affect the risk — the total number of drinks a woman had while pregnant was more predictive of a baby's risk of fetal alcohol syndrome, the researchers found.

During the second half of the first trimester, every one drink per day increase in alcohol intake raised the odds of a certain physical abnormality — in the upper lip — by 25 percent. In contrast, the average number of drinks during the third trimester seemed to affect only the baby's length at birth.

Different mothers, different risk
There are still questions about what factors may influence the risk of fetal alcohol syndrome, Chambers said. "Even if you find 10 women who drink a quart of vodka a day, maybe only five of those babies will have full-blown fetal alcohol syndrome, because there are other factors that influence the risk."

Those factors could include diet, body fat levels, genetic differences, or other environmental exposures, said Ed Riley of San Diego State University, who also studies prenatal exposure to alcohol.

Other studies have found little or no affect on babies born to mothers who drank only occasionally — having one or two drinks a week — throughout pregnancy. But most previous studies relied on mothers' recall of their drinking after birth, whereas in the new study, researchers interviewed women periodically throughout pregnancy.

Riley said the new study adds weight to the argument against drinking any alcohol during pregnancy, and emphasized that the new study showed that any alcohol consumption led to an increased risk. "They showed no threshold effect," he said, "so the more you drank, the greater probability of having an adverse outcome."

Future studies, he said, could follow the same method to look at not only physical abnormalities of the infants, but neurological problems as well. "We know that the brain is very sensitive to prenatal alcohol exposure," he said.

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