Pregnant mothers can drink 4 units of alcohol a week without putting their unborn baby at considerable risk, according to a new study

Expectant mothers have long been cautioned against drinking alcohol during their pregnancy due to an increased risk of birth defects and developmental disabilities — so much so that even occasional drinking while pregnant is often considered to be irresponsible.

According to The Independent, official NHS guidance published only last year said "expectant mothers should not drink at all because 'experts are still unsure exactly how much — if any — alcohol is completely safe for you to have while you’re pregnant.'"

However, a new study suggests that a slight change in attitude towards pregnant women who enjoy the odd glass of wine might be in order.

The new research review, published by BMJ Open, examined 26 existing studies on the effects of light drinking — up to four units a week — during pregnancy, and found little evidence that it caused harm to unborn babies.

One unit is equal to half a pint of beer, lager, or cider at 3.5% strength, a single measure of spirits, or half a glass of wine (175ml) at 11.5% strength — meaning two pints or two glasses of wine a week maximum.

The new study, carried out by experts from the Medical Research Council’s Integrative Epidemiology Unit at the University of Bristol, the University’s School of Social and Community Medicine, and University Hospitals Bristol NHS Foundation Trust, discovered that drinking up to four units a week while pregnant was associated with an average 8% higher risk of giving birth to a slightly smaller baby (SGA), compared to those who did not drink at all.

However, the babies were only two to 14% smaller, and there was "limited evidence for a causal role of light drinking in pregnancy, compared with abstaining, on most of the outcomes examined."

These outcomes included development, behaviour, cognitive impairment, greater problems in pregnancy, or a more complicated delivery. There was a potential risk linked to premature birth, but this outcome was less clear.

The study went on to say that the "distinction between light drinking and abstinence" has been "the point of most tension and confusion for health professionals and pregnant women" and has contributed to "inconsistent guidance and advice now and in the past."

It added: "This issue remains of great public health importance, with alcohol consumption during pregnancy prevalent in the UK, Ireland, New Zealand and Australia with up to 80% of women consuming some alcohol during pregnancy."

However, the researchers said their review showed that the question of light drinking during pregnancy "is not being researched thoroughly enough, if at all."

"The recently proposed change in the guidelines for alcohol use in pregnancy in the UK to complete abstinencewould be an application of the precautionary principle," the study concluded. "This review confirmed some increased risk of babies being born SGA but little direct evidence of any other detrimental effect for maternal drinking up to 32g/week."

"Formulating guidance on the basis of the current evidence is challenging," the study went on. "However, describing the paucity of current research and explaining that 'absence of evidence is not evidence of absence,' appears warranted."