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One at a time best for IVF

Standard in-vitro fertilisation (IVF) treatment should transfer just one embryo at a time, avoiding multiple births and cutting the chance of problems like premature or underweight babies, experts say. The advice is based on new research looking at the results of IVF treatment.

What do we know already?

In the 32 years since the first 'test-tube baby' was born, the science of IVF has progressed considerably. The early years of IVF saw many births of twins and triplets, as researchers transferred multiple embryos into women's wombs, hoping to increase the chances of pregnancy.

We now know that multiple births tend to have worse outcomes for both mothers and babies. Twins and triplets are likely to have more difficult births, are more likely to be born prematurely, and the chances are higher that at least one of the babies will be smaller than usual (less than 2.5 kilograms, which is just over 5 pounds). Premature birth and low birth weight can be fatal for some babies, and lead to life-long health problems for others.

More recently, clinics have been told to transfer no more than two embryos, and many countries have moved to single-embryo transfer, where only one embryo is implanted into the woman's womb.

Even so, the number of multiple births from IVF remains high. In 2006, 20 percent of babies born from IVF in Europe were twins.

It's clear that the chances of pregnancy are higher if more embryos are transferred. But the longer-term success rates – those of healthy, full-term babies – are less certain.

This new research pools the individual results from 1,367 women who took part in eight trials comparing single embryo transfer with double-embryo transfer (transferring two embryos into the womb).

What does the new study say?

As expected, implanting two embryos almost doubled the chance of having a baby: from a 27 in 100 chance of having a baby after a round of single embryo transfer IVF, to a 42 in 100 chance of having a single baby or twins after a double embryo transfer.

However, it also increased the chance of the babies being born prematurely or at a low birth weight, with all the associated health risks. About 30 in 100 babies born from double-embryo transfer were born prematurely, compared with just 13 percent of single-embryo transfer IVF babies. After taking account of other factors like the mothers' age and the quality of the embryos, the risk of prematurity was increased 33 percent by double-embryo transfer. And 24 in 100 double-embryo transfer babies were underweight, compared with just 8 in 100 single-embryo transfer babies.

Also, say the researchers, having two embryos transferred together doesn't increase the chances of a successful pregnancy, compared with having two embryos transferred one after the other. This means having one 'fresh' embryo transferred, then transferring another (frozen, after being created in the first round) if the first doesn't work. In the studies, this process resulted in a baby for 38 in 100 women.

Almost 30 in 100 of the double-embryo transfer IVF rounds resulted in a multiple birth.

The study doesn't give figures for the proportion of multiple births that were full term and normal weight. But the authors say that single births are healthier than multiple births.

How reliable are the findings?

The figures themselves should be reliable. However, all the studies were done in groups of women who had relatively high chances of success from IVF. Most were under 33 and had produced good-quality embryos. So, we can't tell whether these success rates would apply to other women.

Where does the study come from?

The study was done by an international group of researchers from Australia, the UK, Sweden, Finland, Belgium, and the Netherlands. It was published in the BMJ (British Medical Journal) which is published by the British Medical Association.

What does this mean for me?

Most couples undergoing IVF have to pay for it themselves, and the procedure is not cheap. Understandably, most couples are keen to find a clinic with the best success rates.

However, clinics may just give the pregnancy rates or the birth rates. These rates are likely to be higher for clinics that regularly implant two embryos. Yet that may not tell the whole story in terms of which clinics are best at helping couples have healthy, full-term babies.

Experts are now calling for single-embryo implantation to be the standard method for IVF, as it is in many European countries, including Sweden and Finland. It's important for couples contemplating IVF to understand the full picture about what double- or singleembryo transfer involves.

What should I do now?

If you are considering IVF treatment, ask your clinic or doctor about their figures for full-term, healthy weight babies, as well as their pregnancy rates. When considering double- or single-embryo transfer, think about the risks of a multiple pregnancy, as well as the increased chance of pregnancy with double-embryo transfer.


McLernon DJ, Harrild K, Bergh C, et al. Clinical effectiveness of elective single versus double embryo transfer: meta-analysis of individual patient data from randomised trials. BMJ. 2010;341:c6945.

Dec 22, 2010