For women over 40, the chances of a successful birth could be boosted by transferring two embryos

Two embryos are better than one for maximising the chances of an IVF baby, according to one of the largest studies of success rates at fertility clinics.

Transferring two embryos raises the risk of a twin birth – the greatest hazard in fertility treatment – but researchers say that, for older women, the risk could be worth taking.

In the past, clinics transferred multiple embryos in the belief that this would boost the chances of a successful pregnancy and birth. But twin and triplet pregnancies carry a higher risk of premature birth, low birth weight and other hazards. As a result, in 2008 the Human Fertilisation and Embryology Authority (HFEA) ordered IVF clinics to reduce their multiple birth rate, then 24 per cent, to 15 per cent by restricting patients to a single embryo, except where there were medical reasons, such as a recent failed treatment cycle, for transferring two.

The policy has worked and the multiple birth rate is down to 18 per cent for IVF clinics nationally while live birth rates have steadily risen.

But the study, by Professor Debbie Lawlor and colleagues at the Medical Research Council in Bristol, found that women over 40, who have the lowest chance of having a baby, have a low risk of a multiple birth as they are less likely to carry a twin pregnancy to term.

They conclude in The Lancet that “greater freedom” should be given to clinicians and patients “to decide whether to transfer one or two embryos“.

“Our findings provide some support for the transfer of two embryos in women older than 40 years, because the risks of pre-term and low birthweight were lower than those in younger women.”

The researchers added that transferring three embryos should never be attempted, mainly because it increased the risk of severely premature birth. The study analysed more than 33,000 live births from 124,000 cycles of treatment conducted in UK clinics.

The HFEA said clinics were not banned from transferring two embryos but were required to select patients carefully and to demonstrate that they had a “strategy” to reduce their multiple-birth rate.

By freezing spare embryos, fertilised after a single egg collection, and transferring them one at a time should a first attempt fail, the chances of a live birth were increased while the risk of a twin pregnancy was almost eliminated (unless the embryo split to create identical twins), a spokesperson said.

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