Use of cannabis during pregnancy is linked to low birthweight and the need for intensive care, reveals an analysis of the available evidence, published in the online journal BMJ Open.

As cannabis becomes more socially acceptable, it's important that prospective mums-to-be and clinicians are fully up to speed on the potential harms of using the drug during pregnancy, caution the researchers.

Cannabis "remains the drug of choice in developed and developing countries," with up to 5% of 15-64 year olds around the world thought to use it, the researchers point out.

While it never used to be a major cause for concern, recent research has pointed to links between the drug and an increased risk of road traffic accidents, psychosis, HIV, hepatitis, infective endocarditis and TB. Less is known, however, about its possible effects on fetal growth and development.

The researchers carried out a comprehensive trawl of seven research databases for studies published up to 2014, looking at the effects of cannabis use during pregnancy on mother and baby up to six weeks after the birth.

Outcomes, such as anaemia in the mother, birthweight, baby's length, need for neonatal intensive care, head circumference and early birth were all included in the review of 24 studies.

Analysis of the pooled data showed that mums- to-be who used cannabis were 36% more likely to have anaemia than women who didn't use the drug.

Infants exposed to cannabis in the womb were 77% more likely to be under weight at birth and twice as likely to require intensive care as those whose mums had not used cannabis during their pregnancy.

The research in the analysis included observational studies, making it very difficult to draw firm conclusions about cause and effect. And the researchers were not able to extract data on whether any of the study participants used other illicit drugs or how much alcohol they drank--factors known to be associated with a higher risk of low birthweight and premature birth.

Nevertheless, they conclude: "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 the medical and social use of the drug "is rapidly becoming more acceptable in the USA and around the world," understanding its effects on maternal and fetal health "should become a global priority," they suggest.

Women and their doctors could do with more information on the possible harms of using the drug during pregnancy, they say.