A study from Adelaide University has found smoking marijuana around the time of pregnancy more than doubles the chance of a premature birth.

Researchers have released the results of a study into risk factors that affect a woman's likelihood of going into early labour - including smoking marijuana before conception.

It is widely known that smoking cannabis during pregnancy increases the risk of giving birth before 37 weeks, but Professor Claire Roberts from Adelaide University's Robinson Institute says this study is the first to recognise that smoking the drug before conception also has an effect.

"If you think about it, a pregnancy starts with an egg meeting a sperm and the quality or the health of your eggs is going to depend a lot on what you did prior to conception," she said.

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"Certainly other work that we're doing and that other groups are doing is showing that pre-conception is a very important time in determining outcome for pregnancy."

Professor Roberts says women who smoke marijuana are also more likely to smoke cigarettes and that adds to the risk.

"Women who smoke marijuana either prior to or during pregnancy may also do other risky behaviours, like maybe drink alcohol and maybe partake in other illicit drugs," she said.

"What our data analysis has shown, though, is that marijuana has independent effects."

Professor Claire Roberts says marijuana use is particularly harmful for women in the three months before pregnancy.

"We also asked them about things that they were doing during pregnancy, but it turned out that using marijuana prior to pregnancy had a more significant effect than even smoking it during pregnancy," she said.

"Women who smoked marijuana in the three months prior to pregnancy had over 2.3 times the risk of delivering their babies pre-term than women who did not use marijuana prior to pregnancy."

The research team was led by Professor Gus Dekker.

"We already had serious concerns about marijuana because of the mental health issues," he said.

"This is another warning that although people might have a liberal attitude towards marijuana outside pregnancy... if you're really thinking of planning to have a baby, stop it months and months before you try to conceive."

Marijuana was not the only risk factor identified.

"If you are of extreme low weight and it's not part of your normal genetic makeup, like because it's something with your diet - and some young girls now do it for cosmetic reasons, they're really underweight, because they try to be some sort of photo model - that's also not good," Professor Dekker said.

"Try to have a healthy weight, healthy diet. As for many other pregnancy problems, if you're seriously overweight - might be a good time to work on that."

3,000 women studied

More than 3,000 women from Adelaide and Auckland took part in the study which considered a range of things including the woman's health and weight, living arrangements and medical history.

It found where families had a strong history of low birth weight babies, the risk of an early delivery was six times greater.

A family history of pre-eclampsia or diabetes more than doubled the risk.

Professor Roberts says for the first time they also identified two types of premature births.

"We've identified a woman going into labour by her uterus starts contracting and we call that pre-term birth with intact membranes," she said.

"The other type is when the woman has pre-term premature rupture of the membrane, so her waters break and then she goes into labour subsequently."

Up to 10 per cent of pregnancies end in a pre-term birth and Professor Roberts says the rate is increasing.

She says premature babies account for half of all children with cerebral palsy.

Tanya Day's son Ryan, now two-and-a-half years old, was born at 25 weeks gestation and spent the first three months of his life in hospital.

"I had a pregnancy before Ryan and we had a daughter who was born at 24 weeks gestation and she weighed 750 grams," Ms Day said.

"She unfortunately died shortly after birth, so I guess we've had two different outcomes."

She says the health outcomes for premature babies can be quite dire.

"Ryan has cerebral palsy. He had a bleed in his brain when he was about a day old and so that's left him with cerebral palsy now," she said.

"So his left side is somewhat paralysed. He has some developmental delay but he is overall doing really well."

Test for premature births

Ms Day had a series of tests between the two pregnancies but they did not find what caused the early labour.

That is a gap other research being conducted by the Robinson Institute is hoping to fill, by developing a test for premature births.

"We need to add a couple of genetic risk markers and together with the genetic risk markers we hope to get to a good robust predicting test, because we do have some preventative therapies at the moment," Professor Dekker said.

Professor Roberts says they will combine those genetic factors with the clinical and lifestyle issues published today.

"We think we're going to actually have a test within three to five years that will predict women, not only at risk of pre-term birth but other important pregnancy complications that compromise the health of the mother or her baby," she said.

"We think that will absolutely revolutionise antenatal care."

She says it will allow for women to be divided into high and low risk groups and to be given more personalised treatment.