Twin births, triplets and even premature singleton births should be excluded, as these are not a desirable outcome of IVF because of the risk of complications, argues David Healy of Monash University in Clayton.
Many people regard twins or triplets as the ideal result of IVF, providing an instant family, but these pregnancies are much riskier for both mothers and babies, and lead to a higher percentage of children with physical and cognitive disabilities(see below).
Doctors and regulatory agencies are now trying to reduce the soaring number of multiple births worldwide caused by IVF. "Multiple births are now at the highest level ever," says Suzy Leather of the UK's Human Fertilisation and Embryology Authority, which will soon restrict to two the number of embryos clinics can transfer to women under 40. "We want to make healthy single babies the ideal outcome of IVF. The goal is to prevent triplet and twin gestations."
At the moment, however, fertility clinics are usually compared on the basis of their live birth rates, the number of living babies born per transfer or per cycle.
Several governments publish tables listing such figures as a guide for would-be parents, a system that has been attacked by critics who argue that such "league tables" contribute to the rise in multiple births, because they encourage clinics to boost birth rates by transferring more embryos (New Scientist, 13 July 2002, p 4).
So Healy's team is instead proposing a universal measure based on the birth of single, full-term babies.
In principle, this should favour clinics that are technically superior, at the expense of those that try to boost their live birth rate by transferring more embryos.
Hoping to lead by example, Healy's team started off with their clinic's results for 2001. After the 2600 IVF cycles performed in 1860 women there were 448 live births- a live birth rate per cycle of 17 per cent. But when the 120 multiple births were excluded, along with 38 premature singletons, this left 290 births-a success rate of 11 per cent.
The team calls this the BESST rate, for Birth Emphasising a Successful Singleton at Term. "It's the statistic people should measure," Healy says.
Healy's proposal has caught the attention of large fertility societies such as the European Society of Human Reproduction and Embryology, which invited him to write a report for its journal (Human Reproduction, vol 19, p 3). But it is not clear whether any country will adopt BESST.
The US, much criticised in Europe for failing to limit the number of embryos transferred despite having one of the highest rates of multiple births, has already incorporated a new category in its league tables showing the rate of singleton births as a percentage of embryo transfers.
The tables, released a month ago by the Centers for Disease Control in Atlanta, Georgia, highlight the new category so it stands out among other criteria.
"Everybody agreed that was the ideal," says Joyce Zeitz of the Society for Assisted Reproductive Technology in Birmingham, Alabama.
However, the singleton category differs from BESST in several ways. It includes premature births, for instance. In the UK, Leather accepts that existing league tables have failings, and plans to change the way success is represented.
But the agency has not yet decided on a new system. No single measure is perfect, cautions Sean Tipton, spokesman for the American Society for Reproductive Medicine in Birmingham, Alabama.
Even BESST might not reveal a clinic's true quality, as some clinics treat a higher proportion of higher-risk or older patients.
Until there is a way to quantify difficulty of treatment, clinics that admit "easier" patients will appear to be more successful, even if their expertise is poorer, says Tipton. "I don't think we are convinced there is a single better way to do this," he says.
New Scientist issue: 17th January 2004
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Written by Sylvia Pagan Westphal