By Branwen Jeffreys
Health correspondent, BBC News
The number of twins born from IVF needs to be cut because of risks to mothers and babies, a watchdog has said.
IVF often uses more than one embryo
The Human Fertility and Embryology Authority (HFEA) says as treatment has become more successful the number of multiple births has increased.
Some 40% of IVF babies are twins, but many are born prematurely and more than 100 die every year.
The HFEA has begun public consultation on a range of options which could lead to fewer multiple births.
At least half of twins are born prematurely with a lower weight. They are far more likely to need specialist care in the first few months and also are at a greater risk of poor health throughout their lives.
The chances of severe disability such as cerebral palsy are also higher in babies born in twins.
HFEA chairman Shirley Harrison said these risks are avoidable, as are the deaths of some premature babies.
"Doing nothing is not an option. The latest figures show 126 IVF babies die each year because they have been born as twins not single babies. We can't let that continue," she said.
The consultation sets out four options for change, including simply making women more aware of the risks of multiple births.
The HFEA is also considering whether there should be a gradual move towards a maximum for each fertility clinic of 10% of births being twins.
An alternative could be setting out guidelines for which patients should only be given one embryo.
The HFEA says any change needs to be flexible to take account of the wide range of patients seeking fertility treatment.
Older women or those who have already been through several unsuccessful IVF cycles would be very unlikely to be restricted to one embryo transfer.
Susan Morgan is one patient who has benefited from having two embryos transferred.
Twin births can have complications
She now has twin daughters Hannah and Olivia, after many years of stressful and expensive fertility treatment.
"I was very aware of the risks, but it was a numbers game. If I hadn't had multiple embryos I wouldn't have had children," she said.
Most fertility specialists support change and some clinics are already trying to increase the number of patients implanted with just one embryo.
Mr Yacoub Khalaf, the head of the assisted conception unit at Guy's and St Thomas' Trust said: "We are already achieving great successes by replacing only single embryos in many of our suitable patients.
"In the last year our overall pregnancy rate has risen and our multiple pregnancy rate has been reduced."
Despite the consensus it could be hard to bring about change. The vast majority of fertility patients pay for their own treatment as access to NHS funding is extremely patchy.
With an average cost for one cycle of IVF of £5,000 it is no surprise that couples seeking treatment are very interested in success rates.
Each clinic is legally obliged to publish the number of pregnancies and live births per cycle of IVF.
There is a level playing field as the regulations allow a maximum of two embryos to be implanted in women under 40 years old, and a maximum of three for women over 40.
Fertility experts have warned about the combination of patients funding their own treatment and a virtual league table of success rates.
Three years ago, the government said the NHS should be offering one cycle of IVF to suitable patients by April 2005.
It was seen as a stepping stone towards implementing the guidance from the National Institute for Clinical Excellence (Nice) of three NHS funded cycles.
In many areas that simply has not happened, and the criteria for receiving NHS-funded treatment can differ widely from one area to another.
The Department of Health says it is working with the charity Infertility Network to look at the patchy provision of IVF.
A spokesman said: "We recognise that infertility causes pain and distress. It is important that infertile couples have access to IVF regardless of where they live and we would encourage trusts to work towards providing this."