Using marijuana, even before pregnancy, more than doubles a woman's risk of premature birth, according to a new study.

The latest study of more than 3,000 pregnant women in Australia and New Zealand has revealed some of the most common risk factors for having a premature baby.

Babies born at least three weeks before their due date are at an increased risk of having serious and life-threatening health problems and are more likely to develop health problems like heart disease and diabetes in later life.

The research, published today in the journal PLoS ONE, adds to the growing knowledge of risk factors, bringing researchers closer to developing a predictive test.

Lead author Gus Dekker, professor at the University of Adelaide, and his team found that smoking marijuana increased the risk of preterm birth from the average of 7 percent or 8 percent, to between 15 percent and 20 percent.

"It does suggest if you were smoking marijuana, this would be another good reason to stop smoking,'' he said, according to Australian paper The Daily Telegraph. "Marijuana more than doubles your risk and that is marijuana even leading up to pre-pregnancy.'"

Dekker said it was also possible that some of the women in the study continued to smoke marijuana during pregnancy, which may explain why previous studies did not find an association between maternal marijuana use and preterm birth.

"Women are much more open to acknowledge on their booking visit that they were regular users and during their pregnancy they often deny it, although we know that they often still do it,'' he said.

Scientists are still unsure as to whether marijuana use directly causes early delivery.

"We are unable to determine whether this association is due to a toxic effect of marijuana or is a marker of a suite of lifestyle factors that contribute to the risk,'' the authors wrote.

Dekker also identified other risk factors for spontaneous preterm birth such as having a family history of low birth weight babies, having a mother with a history of pre-eclampsia or with type 1 or 2 diabetes and having a history of vaginal bleeds.

Results showed that having a family history of low birth weight babies raises the risk of premature birth by almost six times, and like marijuana use, having a mother with a history of pre-eclampsia or diabetes type 1 or 2 or having a history of vaginal bleeds more than doubles the risk of having a premature birth.

Risk factors involved in the preterm rupture of membranes leading to preterm birth included having mild hypertension, a family history of gestational diabetes, hormonal fertility treatment and a body mass index of less than 20.

Women with mild hypertension that don't require treatment are almost 10 times more likely to have premature birth, having a family history of recurrent gestational increases the risk by eight times, having been treated with hormonal fertility treatment increase the risk by almost four times and having a low BMI more than doubles the risk.

"Our study has found that the risk factors for both forms of preterm birth vary greatly, with a wide variety of health conditions and histories impacting on preterm birth," Dekker said in a statement.

"Better understanding the risk factors involved in preterm birth moves us another step forward in potentially developing a test - genetic or otherwise - that will help us to predict with greater accuracy the risk of preterm birth. Our ultimate aim is to safeguard the lives of babies and their health in the longer term," he concluded.

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