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.