Now experts are calling for women to avoid alcohol completely during pregnancy.
Researchers found that women who drink regular, but limited amounts can cause "subtle, long-term cognitive impairments" in their children.
But these problems may not become evident until years after their birth.
There have been a number of studies already showing that binge and heavy drinking during pregnancy can be dangerous.
But this latest study suggests that even moderate drinking can cause problems.
During the study pregnant rats were given a 2%, 3% or 5% alcohol liquid diet throughout gestation to monitor the area of the brain important not only for learning and memory, but also highly susceptible to prenatal alcohol exposure.
Study author Daniel Savage, a professor at the University of Mexico, said: "One of the continuing concerns of investigators in this field is the general public's apathy or scepticism of reports about the dangers of drinking during pregnancy.
"Yet our findings clearly indicate that even low to moderate levels of drinking during pregnancy cause long-lasting alterations in synaptic plasticity and spatial learning in affected offspring."
He said the new data should act as a wake-up call to women about the dangers of drink and pregnancy.
"Our study's findings should have a major impact on this field of research as well as signal a warning to pregnant women and their family practitioners and obstetricians about the potential danger of moderate drinking during pregnancy."
Charles Goodlett, a professor at Indiana University School of Medicine, said: "A person who binge drinks on a Friday or Saturday night may consume no more alcohol per week than a moderate drinker, but the effects of binge drinking can be as serious, if not more serious, than drinking steadily day after day.
"These are just some of the reasons for why it is difficult to determine if there is a safe level of drinking during pregnancy and why, in the absence of such information, abstinence is the best course of action."
A spokesman for Alcohol Concern said pregnant women should weigh up the evidence before deciding whether to drink.
"The impact of moderate drinking during pregnancy is hard to assess because the research is not all that detailed or comprehensive.
"But if women have any concerns they should steer clear of drinking during pregnancy.
"Women have to weigh up the evidence and make their own decisions."
The report was published in Alcoholism: Clinical and Experimental Research magazine.