Despite the fact that twins are born often — increasingly often with assisted reproduction — doctors have been unclear about what is the best way to deliver pairs of infants.
Is performing a caesarean section the safest option? Is vaginal birth a riskier approach?
It's a question you would think would have been answered by now, but in fact an answer has been lacking. Until now.
The study found that the risks associated with vaginal and C-section births of twins are essentially equal. In other words, vaginal births for twins are as safe as delivering them by caesarean section.
That means women don't need to undergo a C-section for the safety of their babies, said lead author Dr. Jon Barrett, a high-risk pregnancy specialist at Toronto's Sunnybrook Health Sciences Centre.
"If you can get an equally safe result without doing surgery, why would you choose surgery? That's my feeling," Barrett said.
The study enrolled 2,804 women giving birth at 106 centres in 25 countries across North and South America, Europe and the Middle East.
It was funded by the Canadian Institutes of Health Research and is to be presented Thursday at the annual meeting of the Society for Maternal Fetal Medicine in San Francisco.
There are important caveats. The study looked at births from healthy pregnancies, where there was no indication of complications.
The researchers only enrolled women on the verge of delivering, whose first fetus was in the head-down position. So these results don't pertain to women giving birth to twins when the first twin is in the breach position — backside or feet first.
Dr. Kenneth Lim, a high-risk obstetrician at B.C. Children's and Women's Hospital in Vancouver, welcomed the findings and praised the study.
"I think it's going to reassure a lot of women that vaginal birth is perfectly safe in this situation. I think it will reassure a lot of obstetricians. That's a good thing," Lim said.
The study randomly assigned the women to either undergo a C-section or give birth vaginally. Persuading so many women and their doctors to agree to be assigned a delivery method was "a mammoth task," said Lim, who was not involved with the study but was informed of the findings.
"It's very difficult to get people to subject themselves to randomization," said Lim, who is medical director of his hospital's diagnostic and ambulatory program.
"Basically you're saying a computer's going to tell you what to do. Human beings from a Western culture, we're used to controlling our own destiny and making choices for ourselves."
Lim said the issue of twin births has been hotly debated among obstetricians. While his hospital leans towards vaginal births for twins when possible, not all do.
"There is some thinking among caregivers that caesarean might be safer," he said. "That's why the study was done and designed to try to answer this question."
The study looked to see if there were differences in key outcomes between twins born by vaginal delivery compared to those born by C-section. It looked for differences in deaths during delivery and for severe complications for the infants, such as evidence of brain damage.
Fifty-seven of the babies born by C-section had one or more of these outcomes, compared to 52 of the infants born vaginally — or 2.05 per cent versus 1.87 per cent, respectively. Statistically, there is no difference between the two, meaning C-sections neither lower nor raise the risk of death or serious complications in twin births.
Complications were more likely with the second twin born, but there was no difference in rates between the two types of delivery.
Barrett said he isn't surprised by the findings, though he acknowledges others in his field may be.
"My personal bias, from my training and experience, is that provided you're skilled and you've been trained, that you're in the right place with the right support, that you should get results that are excellent," he said.
Barrett admitted he found the increasing trend towards using C-sections to deliver twins frustrating, given that it wasn't based on evidence showing the approach was safer.
Is he hoping this work will stem that tide?
"I do personally feel that it will be nice if unnecessary caesarean sections are not done, but that's not my raison d'etre," Barrett said.
"My raison d'etre, really, is to give accurate information so that when people ask me 'What's the best way to deliver my babies?' I can tell them we now know this is the evidence, that there's no difference between caesarean and vaginal birth."