THE BIRTH of twins would seem like the best possible outcome for any couple undergoing fertility treatment: years of longing for a family rewarded with not just one, but two bundles of joy. It's a scenario that has become increasingly common in the past few decades as the number of multiple births in the UK has risen significantly, mainly thanks to the success of IVF procedures.

But behind the cute images of twins and triplets lie some harsh medical statistics. While the majority of twin babies grow into healthy children, multiple births carry far greater risks and can result in a traumatic start to life, including weeks spent in an intensive care unit. More than 100 IVF babies will die each year as a consequence of being born in a multiple birth and some will be affected by lifelong disabilities, such as cerebral palsy. In addition, the mother is also more likely to suffer dangerous complications during the pregnancy.

Amid these concerns, the fertility watchdog in the UK last week launched a consultation on plans to cut the number of twin pregnancies occurring through IVF treatment. Currently the Human Fertilisation and Embryology Authority (HFEA) code of practice states that women aged under 40 are expected to have no more than two embryos transferred, with women over 40 allowed a maximum of three. Now the focus will be on the use of "single-embryo transfers", where only one embryo will be transferred per IVF cycle.

One of the most radical approaches suggested is the introduction of a 10% cap on the rate of twins for fertility centres in the UK.

But the moves have provoked controversy amid continuing concern at the patchy provision of fertility treatment in the UK. Many couples face long waiting lists and a postcode lottery for treatment, while others are forced to go privately and fork out thousands of pounds for every IVF cycle. And some fear the proposed new restrictions could lessen their chances of having a child even further.

The statistics reveal a significant rise in the number of multiple births in recent years, with fertility treatment thought to be the main factor behind the increase. In 2003, around 18,500 babies were born as part of a multiple birth in the UK, compared with just fewer than 12,000 in 1978. At present, 40% of babies born after IVF are twins, compared with just 1%-2% in the general population.

Aberdeen-based consultant Dr Mark Hamilton is chairman of the British Fertility Society (BFS), which backs reducing the number of embryos implanted simultaneously during IVF treatment.

"If you had medication that was generating a number of side-effects, you perhaps evaluate whether that medication was the right thing to be using," he said. "The analogy with IVF is that maybe in some patients, putting two embryos back is not the wisest course of action."

The BFS is one of 20 organisations - including patient groups and medical colleges - who have developed a consensus statement on the issue. This document notes that high rates of multiple pregnancies from IVF treatment in individual treatment centres "are no longer acceptable" and that single-embryo transfer in those patients deemed to be having a good chance of success should be routine.

Bill Ledger, professor of obstetrics and gynaecology at Sheffield University, points out that the rates of twins in his clinic is around 25%, a figure he describes as "an awful lot", yet not unusual.

"Human beings are meant to have one child at a time, so you are stretching it a bit if the woman has twins, and things do go wrong much more often," he says.

"Most of us who do obstetrics have looked after mums whose babies have been very premature and ended up in intensive care on a ventilator. It is a horribly stressful time and if you can avoid it, you should."

However the HFEA stresses in its consultation that the final outcome of the consultation will definitely not involve a "blanket ban" on two-embryo transfers. Ledger, also a member of Royal College of Obstetricians and Gynaecologists, is keen to point out the single-embryo option is only likely to be used for those younger patients who have the highest chance of success.

"We are not saying every person who has IVF should have one embryo," he explains. "All we are saying is that for women under 35 who have a high chance of getting a baby through IVF anyway, in the first couple of attempts you might have one embryo."

This may sound perfectly reasonable in theory, but in practice a major difficulty is that the provision of IVF treatment is already extremely patchy. In some areas of England, there is little funding for treatment, with couples restricted to only one cycle of IVF or offered none at all. Ledger argues the case that any saving to the NHS from reducing the number of multiple births - for example, through fewer babies being treated in intensive care - should be invested in improving the provision of fertility treatment.

The HFEA consultation says that experiences in other European countries which have adopted single-embryo transfer policies suggests the twin rate has dropped without having an impact on a woman's chance of conceiving. But it also warns it cannot be predicted with certainty if IVF pregnancy rates would "drop slightly" in the UK if more single-embryo transfers were used.

Campaigners point out that, unless provision is improved, some couples could feel their already limited chances for a child will be reduced even further by the new proposals.

Susan Seenan, spokeswoman for Infertility Network UK, says that the situation could be particularly complicated for those who can't get NHS treatment yet might be able to afford to pay for only one cycle of treatment privately.

"If you can only afford to pay for one cycle, then you see twins as the best outcome, even though from a medical perspective that is not always the case," she said.

Seenan points out the situation is better in Scotland, where patients have better access to fertility treatment on the NHS, but warns this does not mean everyone will necessarily welcome the move to single-embryo transfers.

"I think that does make it slightly easier here, but you still have patients saying we are going through all the trauma of treatment and we don't really want to go through that twice," she adds. "That is where people need to be a bit more educated about the risks of a twin pregnancy."

Sarah Caughie, 33, from Peebles, has a five-year-old daughter who was conceived naturally and an eight-week-old son born through privately-funded IVF treatment. She says she would choose a single-embryo transfer if she was going through the treatment again, because of the health risks.

"I think it is something that needs to be done on a case-by-case basis. When people ask me about it, what I say is that if we ever used any of the frozen embryos for treatment, I would insist on just one transfer," she says.

"I had looked into single-embryo transfer before I went through the treatment, but because it was our first time we were told two embryos was the standard procedure and we didn't push it. But I think knowing more about it, it's probably the way I would do it now."

Caughie agrees there should be more discussion around such issues. One difficulty, she points out, is that the main focus is just on getting pregnant in the first place, both for the patient and for the fertility clinics, who are keen to keep their success rates high.

"You are so focused on wanting to get pregnant that you don't think about I want to have a healthy baby', you don't think about that until afterwards. I think that is a big problem," she said.

The HFEA acknowledges that having a "target-driven" culture could bring other difficulties. One concern would be that a clinic may reach its 10% rate halfway through the year, leading to women who could benefit from two-embryo transfer being denied treatment or forced to go elsewhere to find a clinic that is able to treat them.

However, introducing a target is one of only four options the regulatory authority is considering, the "softest" of which is just increasing awareness of the risks of multiple births among clinics and patients and "encouraging" increased use of single-embryo transfers.

The consultation also proposes developing specific criteria around single-embryo transfers which would be enforceable for all clinics or, as another option, introducing this measure only if a clinic goes above a 10% twin rate.

In the early days of IVF it was known to result in triplets, quadruplets or even higher numbers of babies, but now it seems the treatment's association with multiple births could soon be over.

As Ledger points out, it is precisely because IVF treatment has become so successful in recent years that trying to reduce any potential problems of multiple births is even on the agenda.

"In the past, we were just so desperate to get people pregnant that everything else got pushed to the back," he says.

"It is the improvement in technology and skills that now allows us to put the safety of the mum and the child first, rather than just struggling to get anyone pregnant."