Boys are more likely to be born prematurely at 20-24 weeks, a study by researchers at the University of Adelaide has found.

Boy babies are at greater risk of potentially life-threatening pregnancy complications than girls, a new study has found.

Researchers at the University of Adelaide found of these complications nearly 30 per cent of boys were more likely to be born very prematurely.

It's all to do with the genetics of the placenta, say the authors of the study published in the journal PLOS ONE.

Analysis of more than 550,000 births in South Australia over a 30-year period between 1981- 2011 shows there is "very clear" evidence that the sex of the baby has a direct association with pregnancy outcomes.

Baby boys were 27 per cent more likely to be born pre-term at 20-24 weeks gestation.

The risk of a pre-term birth at 30-33 weeks decreased to 24 per cent and to 17 per cent at 34-36 weeks.

Mothers carrying boys are four per cent more likely to suffer gestational diabetes and are also seven-and-a-half per cent more likely to suffer pre-eclampsia, a complication caused by high blood pressure, at full-term.

However, pregnant women carrying a girl had a 22 per cent higher risk for early onset pre-eclampsia requiring a pre-term delivery.

Professor Claire Roberts from the Robinson Research Institute at the University of Adelaide says genetic defects in how the placenta develops are strongly associated with these pregnancy complications.

Previous research has shown that 142 genes are expressed differently in the placenta between the sexes, says Prof Roberts.

"So we think it's the genetic differences in the way the placenta grows and functions that contribute to the differences in risk between boys and girls for these pregnancy complications," she said.

Prof Roberts says its important for mothers to know that genes don't just act alone, they interact with the environment, and there are a number of factors that can affect pregnancy outcomes.

These include maternal smoking, maternal diet and exposure to alcohol and illicit drugs.

Dr Petra Verburg from the University of Groningen in The Netherlands, who helped lead the research, says the results indicate there may be a need for specific interventions tailored to male and female babies.