Scientists have cast doubt on the practice of delivering twins earlier than single babies.
Medical advances mean more babies born prematurely survive
Researchers looked at the birth of 60,443 pairs of twins in the US between 1995 and 1997, the American Journal of Obstetrics and Gynaecology reported.
The team from the University of Ottawa, Canada, found the best time to deliver twins to reduce the risk of death or health problems was at 38 or 39 weeks
In the UK, most twins are delivered between 37 and 38 weeks.
Single pregnancies normally last 40 weeks, but twins often naturally arrive a few weeks early.
If not, doctors will tend to induce twins' birth - as the risk of complications increases from 40 weeks.
In fact, in recent years, doctors in the UK have begun to look to induce twins earlier - at around 37/38 weeks.
The Canadian team found that waiting past 40 weeks increased the risk of the babies dying by 2.5 times, as well as making it more likely babies who did survive would have breathing or heart problems.
But there was little difference between delivering at 37, 38 or 39 weeks for long-term survival.
However twins delivered at 37 weeks were 17% were more likely to need ventilation because of fluid on the lungs than twins born at 39 weeks.
Lead author Jennifer Soucie said: "The study suggests that the optimal date of delivery of twins should be before 40 weeks gestation, and we did not find compelling evidence for being delivered at before 38 weeks."
Patrick O'Brien, a consultant obstetrician at University College London and spokesman for the Royal College of Obstetricians and Gynaecologists, said: "In the UK there has been a trend to deliver twins earlier, around 37 weeks.
"This casts doubt on whether that is the right thing to do. We have always thought there is more risk of problems at 38 weeks, but this does not show that to be the case.
"What we need now is more research to prove this."