Mums to be have been who smoke cannabis during pregnancy have babies that are more likely to be underweight or end up in intensive care.
Experts trawled seven research databases of studies up to 2014 observing the effect the drug had on both mother and child up to six-weeks after the birth.
Using the pooled data, they found that infants exposed to cannabis the womb were 77 per cent more likely to be underweight at birth and twice as likely to require intensive care.
The figures, published in the online journal BMJ Open, also showed that mums-to-be who used cannabis were 36 per cent more likely to have anaemia than women who didn’t.
After alcohol and tobacco, cannabis is the most commonly used drug in the UK.
Less than one in eight adults aged 16 to 59 and less than one in five young people aged
16 to 24 use it the last year.
The observational study was unable to tell whether participants used other illicit drugs or whether they consumed alcohol or smoke during pregnancy.
However the researchers concluded that premature birth, low birthweight, head circumference and increased need for intensive care were the serious consequences of consuming the illicit drug.
Professor John Ehiri of the University of Arizona said: “The effects of in utero exposure to other illicit drugs, such as cocaine, have been widely studied.
“Less is known about the effects of cannabis on foetal growth and development,
or its effects on pregnant women.
“This systematic review summarised and critically appraised the existing literature on the effects of prenatal exposure to cannabis on women and their neonates.
“This meta-analysis demonstrated that women who used cannabis during pregnancy had an increase in the odds of anaemia compared with women who did not use cannabis
“Infants exposed to cannabis in utero had a decrease in birth weight compared with infants whose mothers did not use cannabis during pregnancy.
“Infants exposed to cannabis in utero were also more likely to need placement in the NICU/ICU compared with infants whose mothers avoided using cannabis during pregnancy.”
“There does appear to be negative consequences associated with in utero exposure to cannabis, including a decrease in birthweight and a need for placement in intensive care.
“As use of cannabis gains social acceptance, pregnant women and their medical providers could benefit from health education on potential adverse effects of use of cannabis during pregnancy.”