Women pregnant with twins should elect to give birth at 37 weeks to avoid serious complications, a study from the University of Adelaide has found.

This reinforces commonly held beliefs by New Zealand obstetricians, who advise on 37 to 38 weeks.

The new advice is based on the world's biggest study addressing the timing of birth for women who have an uncomplicated twin pregnancy. Results of the study were published this week in the British Journal of Obstetrics & Gynaecology.

The study included 235 women in Australia, New Zealand and Italy, during which researchers found that babies born to women in the early birth group (37 weeks) were significantly less likely to be small for their gestational age compared with babies born to women in the standard care group (38 weeks or later).

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Lead researcher Professor Jodie Dodd said, infants of a twin pregnancy are recognised to be at risk of problems during pregnancy, particularly from a slowing of the rate of growth in one or both twins.

"This slowing of the growth rate can result in low birth weight, which is associated with an increased need for care in the neonatal nursery in the short term and increased risk of health problems in later life, including heart disease and diabetes. There is also the risk of one or both twins being stillborn.

"This is why we've taken such a great interest in the optimal time for twins' birth," she said.

"We found that at 37 weeks, elective birth is associated with a significant reduction in the risk of serious morbidity for infants, without increasing complications related to immaturity or induction of labour."

Professor Dodd said there has been a lot of uncertainty in clinical practice about the optimal time for twins' birth.

"We hope this study will help clinicians to make recommendations to women with healthy twin pregnancies that lead to less complications at birth, and therefore lead to happier, healthier lives for their babies.

The President of the New Zealand Multiple Birth Association Fiona Purchas said the reports' findings are common knowledge and standard practice in New Zealand.

"Nearly every multiple mother that you talk to - unless their babies were born prematurely and came early - they would have had their babies at between 37 and 38 weeks."

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Ms Purchas said the report gives evidence to what obstetricians have been recommending for a long time.

For most women it is recommended that if they haven't delivered by 38 weeks that the babies are induced at that stage, said Ms Purchas.

"Research does show that after 38 weeks for multiples the risks are increased."

Unlike for single births, where the expected birth is at 40 weeks, full term for multiples is 37 to 38 weeks, she said.

"It's not early and in fact the outcome for multiples are much better than for babies that are left longer than that."

Herself a mother of twins, Ms Purchas said that she had an induction planned at 38 weeks but became quite unwell with the flu and ended up giving birth at 40 weeks.

"I was very closely monitored," she said. "At 40 weeks, it was pretty tough."

The president of the Auckland Central Multiple Birth Club, Nicole Francis said she found the research fascinating.

"And we also advised to finish work a lot earlier as well - anything from 28 weeks to 32 weeks."

Ms Francis gave birth to her identical twin daughters Sharna and Alexia - now 22-months-old - at 36 weeks.

"We went for a caesar [caesarean section] one of my girls' growth had significantly decreased and so at 34 weeks I was on bed rest in hospital.

"Once we hit the 36 mark they said that's enough, they need to come out now," said Ms Francis.

"I think as a pregnant twin mother I would never, ever want to get past 38 weeks anyway because you're so uncomfortable."

The study was supported by a grant from the Australian Women's and Children's Hospital Foundation.