Strict government guidelines warning pregnant women against drinking any alcohol are not justified by evidence, a landmark study has found.
There is little evidence having the occasional drink while pregnant does any harm to a baby, a review has concluded.
"We know that alcohol can cause harm both at a cellular level and a clinical level therefore the precautionary approach is safest and one of the reasons is that often people who are given the go-ahead to drink will drink more than they are advised to drink".
The study carried out a systematic review of 5,000 studies and selected 26 which looked at the impact of light drinking of two units up to twice a week, or four units a week, equivalent to a total of around 32 g compared with no alcohol at all.
It found "some evidence" that drinking up to four units of alcohol per week may be associated with a higher risk of having a smaller baby or giving birth prematurely-but nothing conclusive.
These include miscarriage, premature birth, undersized babies, and longer-term issues, such as the developmental delays, impaired intellect, and behavioural difficulties typical of fetal alcohol syndrome, the study in the "British Medical Journal Open" revealed.
A spokeswoman for the British Pregnancy Advisory Service (BPAS) said: "What seems to lie at the heart of public messages addressing alcohol in pregnancy is whether women can be trusted to understand the existing evidence, and whether they are able to recognise the difference between light and heavy drinking".
The advice backs the guidelines from the HSE, which recommends abstinence.
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"While this study adds to the evidence that drinking one to two units of alcohol a week after the first 12 weeks of pregnancy is unlikely to have a harmful impact on the baby or pregnancy, we can not rule out the risks altogether".
It compared this to no alcohol at all.
But they said while there was an association, this did not prove a direct cause of smaller babies at birth. There was also a potential risk linked to premature birth, although this was less clear.
While it does not say light drinking is safe, the research does highlight the weak evidence on which government advice is based, they said.
The evidence on how much, if any, is safe to drink, or at what stages of pregnancy, is notable by its absence, they add.
For most of the outcomes the researchers analysed, there were only a few studies that compared light to non-drinkers.
Until recently United Kingdom guidelines advised women to avoid drinking alcohol while trying to conceive, and in the first trimester.
But this has left many women confused about whether there a safe limit and if just one glass was harmful.