Marijuana use in pregnancy is major risk for preterm birth

University of Adelaide

International research led by the University of Adelaide has for the first time shown a direct link between continued marijuana use during pregnancy and pre-term birth.

The study evaluated data from more than 5500 pregnant women from Australia, New Zealand, Ireland and the United Kingdom who took part in the SCOPE (SCreening fOr Pregnancy Endpoints) study. Of those women, 5.6% reported using marijuana before or during pregnancy.

A research team led by the University of Adelaide's Robinson Research Institute 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, published online ahead of print in the journal Reproductive Toxicology, 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.

"Our results suggest that more than 6% of pre-term births could have been prevented if women did not use marijuana during pregnancy, irrespective of other risk factors," says lead author Professor Claire Roberts from the University's Robinson Research Institute.

"In the Australian participating center, which has a higher proportion of pregnant marijuana users, almost 12% of pre-term births could have been prevented.

"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."

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% of marijuana users having delivered at less than 28 weeks' gestation and 64% at less than 32 weeks, compared with non-users: 5% at 28 weeks' gestation and 16% at 32 weeks;
• The Australian participating center had the highest rate of women using marijuana before or during pregnancy (12%), followed by New Zealand (5%), Ireland (4%) and the United Kingdom (4%).

Professor Roberts says these results have implications for pregnant women the world over.
"In the United States, 23 states have now legalized marijuana in some form, either for medicinal or recreational use. Canada's government has also promised to decriminalize marijuana for recreational use. Such widespread legalization of marijuana raises concerns about its safety for pregnant women.

"Anecdotally, we know that some women are using marijuana to reduce nausea in pregnancy, even though there is no medical evidence to support this. 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 says.

This study has been funded by the National Health and Medical Research Council (NHMRC) and the South Australian Premier's Science and Research Fund.