Multiple pregnancies have been very common among women who have undergone IVF (in vitro fertilisation), or the ICSI (intracytoplasmic sperm injection) technique in which a sperm is injected directly into an egg.

A third of all IVF/ICSI pregnancies were at one stage multiple, compared to a rate of under one per cent for natural conception.

This is because doctors tried to maximise conception chances by replacing two, three or even more embryos into the womb.

Often they are under pressure from the prospective parents who cannot afford repeated cycles of treatment, and from their own laboratories who are in competition with other centres to produce high success rates.

Limiting the number of replaced embryos to two or three had led to a decline in triplets, but doctors have been unable to reduce the number of twin pregnancies.

Twin pregnancies can lead to serious complications, including smaller babies, premature birth and obstetrical problems.

The difficulty is that doctors have been unable to identify embryos with a high potential for successful implantation in the womb.

But researchers at Middelheim Hospital, Antwerp, Belgium, have now identified characteristics which can predict the likelihood that an embryo will be successfully implanted. These include the speed of cell division within the embryo and the regularity of its shape.

Theory produced results

Scientists can predict which embyos are likely to implant into the womb

They tested their theory on women under 34 who were undergoing their first IVF or ICSI treatment cycle.

They achieved ten successful pregnancies among 26 women who received just one replaced embryo - a success rate of 40%.

A pregnancy rate of 25%-30% per menstrual cycle is the norm for fertile couples.

Among 27 women who received two embryos, there were 19 successful pregnancies, but six of these were twin pregnancies.

The researchers concluded that the success rate for those women who received a single embryo was sufficiently high to justify its use wherever appropriate.

Researcher Dr Jan Gerris, who presented the study at the European Society of Human Reproduction and Embryology annual meeting in Tours, France, on Monday, said: "Single embryo transfer will progressively be considered the standard of good medical practice in all women under 38 in their first IVF/ICSI attempt.

"As roughly 70% of all IVF/ICSI cycles produce at least one top quality embryo, the incidence of twins can be reduced drastically."

Professor Ian Craft, director of the London Fertility Centre, said it would be an advance if most people could achieve a successful pregnancy using just one replanted embryo.

"Human beings are made to have single babies. The risk of having a single baby is far less than having twins."

But he added that guaranteeing a successful pregnancy might never be possible among older women whose fertility was diminished, and who seldom had multiple pregnancies anyway.

Mr Sam Abdalla, director of the IVF clinic at The Lister Hospital, in London, said: "At the moment we transfer two or three embryos because we do not know which will result in a pregnancy, and we are trying to strike a balance between the risk of multiple pregnancy and the desire of the woman to get pregnant.