A new study by the Coombe Women's Hospital in Dublin has found that 60% of expectant mothers consume alcohol during pregnancy.
The problem is highest among younger women, with more than two-thirds of new mothers under the age of 18 admitting that they drank while pregnant.
Of those who drink more than 10 units per week, the highest percentage was found to be in the 18-24 age group.
The Coombe says Irish and UK-born mothers are more likely to drink more than six units of alcohol per week than those from other countries.
Elsewhere, the study says more than 45% of smokers admit that they do not quit during pregnancy, with the problem also found to be highest among young mothers.
The authors say the findings are a cause for concern and a sign that more needs to be done to help women make healthier choices when they are expecting.