Continued marijuana use during pregnancy may increase the risk of pre-term birth by about five times, a new study has found for the first time.
The study evaluated data from more than 5,500 pregnant women from Australia, New Zealand, Ireland and the UK who took part in the SCOPE (SCreening fOr Pregnancy Endpoints) study. Of those women, 5.6 per cent reported using marijuana before or during pregnancy.
The researchers considered a range of risk factors - such as cigarette smoking, age, obesity and socio-economic status - and their links to serious pregnancy complications.
The results show that once all other major risk factors have been accounted for, continued marijuana use through to 20 weeks' gestation is independently associated with a five-fold increase in the risk of pre-term birth, researchers said.
"Our results suggest that more than 6 per cent of pre-term births could have been prevented if women did not use marijuana during pregnancy, irrespective of other risk factors," said lead author Claire Roberts, professor at the University of Adelaide in Australia.
"This is the first time that continued marijuana use in pregnancy has been independently linked to pre-term birth. Based on our findings, we consider marijuana to be a major public health concern for pregnant women and their babies," she said.
The study found that among the 236 pre-term births recorded in the group, women who continued to use marijuana at 20 weeks' gestation had a significantly shorter gestation (just less than 30 weeks on average) compared with those who did not use marijuana (more than 34 weeks on average).
The proportion of very early pre-term birth was also higher, with 36 per cent of marijuana users having delivered at less than 28 weeks' gestation and 64 per cent at less than 32 weeks, compared with non-users: 5 per cent at 28 weeks' gestation and 16 per cent at 32 weeks.
"Anecdotally, we know that some women are using marijuana to reduce nausea in pregnancy, even though there is no medical evidence to support this," Roberts said.
"Our study was unable to determine whether there is a 'safe' time prior to 20 weeks' gestation to give up marijuana. Therefore, we recommend total abstinence from marijuana during pregnancy," she said.
The study appears in the journal Reproductive Toxicology.
