Women trying for a baby and those in the first three months of pregnancy should not drink any alcohol, updated UK guidelines say.
The Royal College of Obstetricians and Gynaecologists (RCOG) had previously said a couple of glasses of wine a week was acceptable.
It now says abstinence is the only way to be certain that the baby is not harmed.
There is no proven safe amount that women can drink during pregnancy.
The updated advice now chimes with guidelines from the National Institute for Health and Care Excellence (NICE).
In the US, experts say there is no safe time to drink during pregnancy.
But the RCOG highlights around the time of conception and the first three months of pregnancy as the most risky.
Alcohol in pregnancy
Drinking alcohol may affect the unborn baby as some will pass through the placenta.
Around conception and during the first three months, it may increase the chance of miscarriage, says the RCOG.
After this time, women are advised to not drink more than one to two units, more than once or twice a week, it says.
Drinking more than this could affect the development of the baby, in particular the way the baby's brain develops and the way the baby grows in the womb, which can lead to foetal growth restriction and increase the risk of stillbirth and premature labour, says the advice.
What is a unit of alcohol?
- One unit of alcohol is about half a pint of bitter or 4.5% lager, or a single measure of spirits (25ml).
- A 175ml glass of 12% wine is 2.1 units and a pint of strong beer (ABV 5.2%) is three units
Philippa Marsden, of the RCOG, said: "For women planning a family, it is advisable not to drink during this time. Either partner drinking heavily can make it more difficult to conceive.
"During early pregnancy, the safest approach is to abstain from alcohol and after the first trimester keep within the recommended amounts if you do decide to have an alcohol drink. The same applies for women who decide to breastfeed.
"If you cut down or stop drinking at any point during pregnancy, it can make a difference to your baby. However, in some instances, once the damage has been done, it cannot be reversed. If you have any questions or concerns about alcohol consumption talk to your midwife, GP or health visitor who can offer support and advice."
Dr Simon Newell, of the Royal College of Paediatrics and Child Health, said: "There are lots of mixed messages when it comes to alcohol advice so today's guidance is a welcome and reliable source of information for women who are thinking about trying for a baby and for women who have already become pregnant."
He said about 6,000 babies a year in the UK are born with some form of damage as a direct result of alcohol.
"It is impossible to say what constitutes a 'safe' amount of alcohol a mother can drink as every pregnancy is different, so our advice to mothers is don't take a chance with your baby's health and drink no alcohol at all," he said.
Ann Furedi, chief executive of the British Pregnancy Advisory Service, was concerned that the advice might cause some women undue anxiety.
"This guidance takes a precautionary approach to women drinking alcohol in pregnancy. It may be wise to avoid alcohol when planning a baby, but the fact is many pregnancies are not planned.
"We should reassure women that if they have had an episode of binge drinking before they found out they were pregnant, they really should not worry. It is very troubling to see women so concerned about the damage they have caused their baby they consider ending what would otherwise be a wanted pregnancy, when there's no need for such anxiety.
"This guidance also makes clear that after the first three months, there is no evidence of harm to the baby at low levels of alcohol, so women who like to relax with a glass of wine once or twice a week should not feel guilty about doing so."