Twins should be delivered when an expectant mother is 37 weeks pregnant to reduce the risk of the babies dying, experts have said.
Researchers examined data on more than 35,000 twin pregnancies to determine the best time for delivery.
Twins or triplets hold a higher risk of stillbirth or neonatal death. Mothers expecting multiple babies are often told their babies must be delivered early to reduce the risk.
But the optimal age for when twins or triplets should be born was not known.
The international team of researchers examined 32 studies of women with uncomplicated twin pregnancies that reported rates of stillbirth and neonatal mortality after 34 weeks.
The study, published in The BMJ, found that dichorionic pregnancies - twins who have individual placentas - should be delivered at 37 weeks, as delay by just a week led to an additional 8.8 deaths per 1,000.
In monochorionic pregnancies - twins that share the same placenta - "there is no clear evidence to support routine delivery before 36 weeks' gestation", the authors found.
They concluded: "To minimise perinatal deaths, in uncomplicated dichorionic twin pregnancies delivery should be considered at 37 weeks' gestation; in monochorionic pregnancies delivery should be considered at 36 weeks."
Keith Reed, chief executive of the Twins and Multiple Births Association (Tamba), said: "A study of this size and with these significant findings has never been published in the UK before.
"The risk of stillbirth is higher in twin pregnancies than in singleton pregnancies, so any new information which could save the lives of more babies in multiple pregnancies is obviously an exciting step forward."
Mark Kilby, professor of foetal medicine and spokesman for the Royal College of Obstetricians and Gynaecologists, said: "Although multiple births account for just 3% of all births in the UK, they made up just under 10% of all stillbirths in England, Wales and Scotland in 2014.
"With any twin pregnancy, healthcare professionals should provide closer surveillance in order to ensure early diagnosis and treatment of possible complications. In addition, the risks and benefits of different delivery options and timings should be discussed so a woman can make informed decisions about her birth plan where possible."