It’s known that drinking during pregnancy leaves babies vulnerable to a spectrum of abnormalities called fetal alcohol syndrome. Now, a new study pinpoints the latter half of the first trimester as a critical time in the development of some of the syndrome’s most telling physical characteristics.

Study authors also stressed that their research illustrates there is no safe amount of drinking during pregnancy, since the amount of drinking that produced these features in infants varied from woman to woman.

“The fact that we didn’t find a safe threshold is important,” said study author Christina Chambers, an associate professor of pediatrics and family and preventive medicine at the University of California, San Diego. “Not every child of women who drink even very heavily has all the features, so there are certain susceptibility factors that we don’t know.”