By Caroline Parkinson
Health reporter, BBC News, in Barcelona
Most experts believe twins carry a greater risk of complications
A US doctor has sparked controversy by arguing couples undergoing IVF are being "misled" about the increased risks of twin pregnancies.
Dr Norbert Gleicher, from New York's Center for Human Reproduction, told a European fertility conference the risks had been over-stated.
He said people wanting two babies should be told one twin pregnancy was no riskier than two single pregnancies.
But the majority of fertility doctors rejected his views.
They said twin pregnancies carried significant risks to both mother and child.
The UK fertility regulator, the Human Fertilisation and Embryology Authority, is currently asking clinics to cut their rate of multiple pregnancies to about 10% by the end of 2012.
The current national average is 24%.
To do this, they will need to increase the number of cases where women have only one embryo transferred in a process called single embryo transfer (SET).
Experts believe a move to single embryo transfer in most cases is warranted, to reduce the risk of premature birth, low birth weight, cerebral palsy and risks to the mother such as pre-eclampsia seen in twin pregnancies.
Up to 60% of IVF twins have to be transferred to an intensive care unit after birth, and twins are six times more likely to die in the first year of life than singletons, research suggests.
But Dr Gleicher said at the European Society of Human Reproduction and Embryology (ESHRE) in Barcelona: "In my opinion patients are being misled and put to unnecessary expense."
"I think it's categorically wrong."
He said the risks of twin pregnancies had been over-estimated because they were compared with one single baby pregnancy, rather than the two a couple would require to get two babies.
He argued IVF pregnancies were 40% safer than natural twin pregnancies, and that this should be taken into account when previous research was being examined.
In addition, he argued two pregnancies meant more attempts at IVF, with all the extra treatment and anxiety involved.
Achieving a second pregnancy may also take more than one attempt.
Dr Gleicher, whose study will appear in the journal Fertility and Sterility, said: "This is an example of how the medical profession, while meaning very well, can get dramatically misguided.
"When you compare twins with two consecutive singleton deliveries the excess risk for twins disappears.
"And since this is the principle argument for single embryo transfer you seriously have to question the concept of single embryo transfer."
But three leading experts - Professor Peter Braude, ESHRE chairman Professor Joep Geraedts, and Dr Francoise Shenfield, a member of ESHRE's executive committee all signed a statement criticising Dr Gleicher's views.
They said: "Whatever the hypothesis, which may be interesting for doctors to mull over and discuss, there are significant risks to multiple pregnancies, and we should not be generating them deliberately.
"IVF babies also deserve the best start in life."
They warned twin pregnancies carry two to three times the risk of pre-eclampsia and post birth haemorrhage, and that babies are between five and seven times more likely to be born with cerebral palsy if they are a twin.
Latest figures show that in 2005, a total of 11,268 children were born in the UK following IVF.
Of these, 61% were from single pregnancies, 38% from twin pregnancies, and 1% from triplet pregnancies.