Using cannabis during pregnancy ‘doubles risk’ of having underweight baby, scientists say

Researchers say expectant mothers should be aware of heightened risks, reports Lamiat Sabin

Previous studies have provided enough evidence to allow solid conclusions to be drawn about the use of cannabis during pregnancy (Getty)

Women who use cannabis while pregnant double the risk of stunting the growth of their babies, researchers have found.

Nearly 60,000 expectant mothers were part of the largest study to date of its kind, which found that cannabis users were more likely to give birth to smaller and underweight babies.

Researchers say they also found a direct link between exposure to marijuana during pregnancy and a premature birth, as well as a heightened risk of the newborn having to be admitted to a neonatal intensive care unit.

A low score on the Apgar test – a quick method of assessing a baby’s health minutes after they are born – was also more common compared to babies born of women who did not use cannabis during pregnancy.

Humans and many animals are born with cannabinoid receptors, as a body creates its own versions of cannabis compounds called endocannabinoids.

But scientists said that a developing foetus exposed to exogenous cannabinoids, such as THC – which makes people who smoke or ingest cannabis feel “high”, is at increased risk of “changes in the prefrontal cortex and theoretically with its development and function”.

Previous studies did not have enough evidence to allow solid conclusions to be drawn about the use of marijuana during pregnancy and the health of the babies, they added.

Women who use medicinal cannabis to alleviate symptoms of pregnancy-related nausea, as well as anxiety and chronic pain – or smoke it recreationally – should be aware of the higher risks, the researchers said.

Author Dr Greg Marchand, of the Marchand Institute for Minimally Invasive Surgery in the US, said: “This study found that women exposed to marijuana in pregnancy were at a significantly increased risk of adverse neonatal outcomes.”

He added: “These findings suggest that increasing awareness about these risks may be associated with improved outcomes.”

Up to 5 per cent of women report using marijuana during pregnancy.

But some studies have reported that between 15 to 28 per cent of young expectant mothers in socio-economically deprived areas use it.

Due to these cases being self-reported, the true proportion of marijuana use could be much higher.

Expectant mothers who use cannabis were twice as likely to have babies weighing under 5lbs, 8oz (2.5kg) – which can lead to serious health problems, the study found. An average newborn weighs 8lbs.

The number of babies small for their gestational age or were born prematurely rose by 61 and 28 per cent respectively when cannabis was used during pregnancy. Neonatal intensive care unit admissions rose by 38 per cent.

Apgar scores – that assess pulse, breathing and reactions – fell by 26 per cent, the study also found.

Dr Marchand said: “Exposure to cannabis in the womb may be associated with changes in the prefrontal cortex – and theoretically with its development and function.”

The pre-frontal cortex is an area of the brain that controls complex cognitive behaviour including attention, decision-making, moderating social behaviour, and memory.

Cannabis may change a foetus’ biology, Dr Marchand said, after psychoactive compounds – including the primary one, THC – transfers from mother to baby via the placenta.

Dr Marchand said: “Remarkably, 34 to 60 per cent of individuals who use marijuana keep using it during pregnancy.

“Many women cite the belief that marijuana use is relatively safe during pregnancy among other reasons for continuing use.

“Given increasing marijuana legalisation and use worldwide, raising awareness and educating patients about these adverse outcomes may help to improve neonatal health.”

While increases in several adverse pregnancy outcomes among women who use cannabis were found in the study, there was no finding of any connection between marijuana use and birth defects.

Dr Marchand’s team pooled data from 16 studies across the US, Canada and Jamaica. The eight-month-long study involved 14 authors, including obstetricians, pediatricians, researchers, and epidemiologists.

The report, titled ‘Birth Outcomes of Neonates exposed to Marijuana in Utero: A Systematic Review and Meta-analysis’, was published this week on the JAMA (Journal American Medical Association) Network Open website.