Having twins as a result of in vitro fertilisation (IVF) carries higher risks of complications for both mother and babies than having two single babies from separate IVF procedures, according to a new Swedish study.
The extra concerns that come with multiple births are nothing new. But even as many fertility clinics have stopped regularly implanting more than one embryo, debate has continued over whether having twins through IVF is really a bad thing for couples desperate for children.
"A lot of patients, when they've had infertility for a long time, think that it's a bonus to get two," said Dr Lynn Westphal, a women's health and fertility specialist at Stanford University Medical Center in Palo Alto, California.
But, "We know it's always safest to have one child at a time," said Westphal, who wasn't involved in the new study.
Researchers led by Dr Antonina Sazonova of Sahlgrenska University Hospital in Gothenburg analysed data from fresh and frozen embryo transfers done at Swedish IVF clinics between 2002 and 2006. Those records included 991 women who ultimately gave birth to twins after a double embryo transfer and 921 mothers with two kids born through separate rounds of IVF.
Almost 47 per cent of twin babies were born prematurely and 39 per cent were considered low birth weight, Sazonova's team reported in the journal Fertility and Sterility. That compared to the seven per cent of singleton babies that were preemies and less than five per cent born small.
Twins were also more likely to have breathing complications, sepsis or jaundice. And their mothers had two to three times more preeclampsia - high blood pressure and protein in the urine - and were four times more likely to need a C-section than women who had two single births.
However, there was no difference in the babies' chances of having severe malformations and other life-threatening health problems.
Westphal said she and her colleagues have been counselling more and more women to have a single embryo implanted, especially the younger, healthier fertility patients.
"Obviously the acceptance of that is difficult; patients may be less likely to want to do that especially if they're paying for their treatment," she added.
A typical IVF cycle runs for US$12,000 to $18,000 and may or may not be covered by insurance in the US. In Sweden, IVF is publicly funded for infertile couples - but only for one live birth, the research team noted.
Some data suggest women are just as likely to get pregnant if they have a single embryo implanted during a round of fertility treatment, versus more than one. But findings have been inconsistent, according to Westphal.
"A lot of patients are focusing just on getting pregnant - they're not looking at the whole pregnancy and looking at the outcome of twins," she said.
"They're just thinking if they transfer more, they're more likely to get pregnant. And they're thinking if they have twins, they're just getting everything done at one time."
But the new findings argue against that approach.
The researchers called outcomes for both babies and mothers "dramatically better" for women who had two children through separate IVF pregnancies, compared to those who had twins.
"These results support single-embryo transfer to minimise the risks associated with twin pregnancies," they wrote.