More than half of women drink more than the recommended limits during the first three months of pregnancy, a study suggests.
Some 53% of women drank more than the upper limit of two units a week during the first trimester, figures show.
Middle class women are more likely than women from other classes to drink more than the recommended limits during pregnancy, researchers said.
They called for health officials to revise their guidance on drinking in pregnancy after their study showed that even those adhering to the suggested limits are more likely to have problems with their babies than those who do not drink at all.
The study, published in the Journal of Epidemiology and Community Health, examined 1,200 women in Leeds who filled out food frequency questionnaires , including information on alcohol consumption during the month before conception and the three trimesters of pregnancy.
They noted that alcohol intake was "significantly higher" before conception, and during the first three months of pregnancy than in the last two trimesters.
On average women who participated in the study drank 11.2 units a week before conception and 4 units a week during the first three months and less than two a week during the the last six months of pregnancy.
Those who drank more than two units a week were more likely to be older, educated to degree level and more likely to live in affluent areas, they found.
Of all of the babies born, 13% were underweight, 4.4% were classed as being of a "low birth weight" and 4.3% were born prematurely.
During pregnancy alcohol consumption was associated with a 100g reduction in birth weight for women consuming more than two units a week during the first trimester, they found.
If a woman drank more than two units a week during the first three months of pregnancy they are twice as likely to have a baby who was born "small for gestational age" when compared to non-drinkers, the researchers found.
They added: " A similar association was observed for preterm birth, where even women who adhered to the... guidance limiting alcohol to no more than two units per week were at risk of having babies born preterm compared to women who abstained from alcohol."
In light of the findings, the researchers called on health officials to revise their guidance on drinking in pregnancy,
NHS guidance says that women should avoid drinking alcohol if they are pregnant or trying to conceive. If they do drink they should have no more than " one to two units of alcohol once or twice a week".
But the study's authors said that the results highlight the need for endorsing the " abstinence -only" message.
"Our ?ndings suggest that women should be advised to abstain from alcohol when planning to conceive and throughout pregnancy," they wrote.
"We found the ?rst trimester to be the period most sensitive to the effect of alcohol on the developing foetus. Women adhering to guidelines in this period were still at increased risk of adverse birth outcomes.
"Our results highlight the need for endorsing the abstinence-only message, and further illuminate how timing of exposure is important in the association of alcohol with birth outcomes, with the first trimester being the most vulnerable period."
Commenting on the findings, Dr Ron Gray, senior clinical research fellow at the University of Oxford, said further research was needed on whether there is any completely safe amount to drink during pregnancy.
He said: " The authors of this new study correctly point out there is conflicting evidence on the foetal effects of consuming up to two units of alcohol per week during pregnancy, but their study does not really help to resolve this conflict.
"The authors investigated alcohol consumption during each trimester of pregnancy and tried to link it to five different birth outcomes (including birth weight and preterm birth). They present data on 1,135 women in the first trimester, 808 women in the second and 384 in the third. These numbers are relatively small for these kinds of studies. This means that the results are less precise than might be achieved with a much larger study. Also, given the high number of different statistical tests performed (around 40) one might expect a certain number of the results to be significant just by chance."
Professor Andrew Whitelaw, professor of neonatal medicine at the University of Bristol, added: "This is further evidence that even moderate amounts of alcohol are toxic to the growing foetus and direct toxicity is further worsened by the increased complications of premature birth.
"As 38% of the women admitted risky drinking (over 10 units/week) before pregnancy and pregnancy is sometimes discovered late, the advice has to be to avoid alcohol completely when trying to conceive."
Dr Patrick O'Brien, spokesman for the Royal College of Obstetricians and Gynaecologists (RCOG), said: "The findings suggest that even small amounts of alcohol in the first three months of pregnancy may increase the risk of a small impairment to the baby's growth and possibly premature birth.
"This is consistent with RCOG advice that if a woman falls pregnant she should abstain from alcohol during the first 12 weeks of pregnancy because this is a particularly sensitive time for the baby's development.
"However, while the safest approach would be to choose not to drink at all, small amounts of alcohol, not more than one to two units once or twice a week, have not been shown to be harmful after 12 weeks of pregnancy.
"Pregnant women should always consult their midwives or doctors if they have any concerns about their alcohol intake."
A Department of Health spokeswoman said: "Drinking during pregnancy can be associated with miscarriage, foetal alcohol syndrome and low birth weight.
"Our advice remains that women who are trying to conceive or are pregnant should avoid alcohol - especially in the first trimester. If women choose to drink, to minimise the risk to the baby, they should not drink more than one to two units of alcohol once or twice a week and should not get drunk."