By Michelle Roberts
BBC News, Health reporter in Lyon
IVF ups the risk of a twin pregnancy even if only one embryo is transferred, claim scientists.
Time lapse equipment was used to follow progress
Japanese researchers told a fertility conference in Lyon how the process of IVF encourages embryos to split into identical twins.
There has been mounting pressure to cut the IVF twin rate because of a raised health risk to mothers and babies.
Some say single embryo transfer is the answer, but the latest work suggests it may instead be part of the problem.
Currently, 40% of IVF babies are twins, mostly because more than one embryo is transferred to maximise the chance of a successful pregnancy.
But many of these babies are born prematurely and more than 100 die every year.
The UK's fertility watchdog, the Human Fertilisation and Embryology Authority, has been consulting the public on a range of options which could lead to fewer multiple births, including single embryo transfer.
But the chance of a successful pregnancy is lower with single embryo transfer.
So researchers have been looking at developing the embryo for longer in the lab - until the blastocyst stage when it is a ball of many cells - before transferring it to the womb to maximise the chance of implantation.
Time lapse technology
Ms Dianne Payne and colleagues at the Mio Fertility Clinic used special "time-lapse" computer software to observe minute by minute what was happening to single IVF embryos as they grew in the lab.
Two of 26 blastocysts developed obvious signs that they had the potential to be identical or monozygotic twins - two distinct inner cell masses (ICMs).
The cause was changes in the lab, called blastocyst collapse, which the researchers believe are directly related to IVF.
Ms Payne said: "It may happen because we are keeping the embryo in culture for longer until it has reached the blastocyst stage.
"And the formation of two ICMs during blastocyst development may be the cause of the high twin rate after extended culture."
She said it should be possible for doctors to examine embryos before transplantation and advise patients about any potential twinning risk.
Mr Richard Kennedy, chairman of the British Fertility Society, said: "The use of blastocyst transfer is increasing as we are being pushed towards elective single embryo transfer to reduce multiple pregnancies.
"This study is very interesting and clearly indicates an area of concern that needs further investigating.
"Although it is not the major contributor to twinning, it is important and we need to monitor this outcome."
Professor Alison Murdoch, professor of reproductive medicine at the Newcastle Fertility Centre At Life, said: "We need to take careful account of this new information.
"Blastocyst transfer may be associated with an increase in the risk of identical twins and this work offers an explanation for why this might be happening.
"The HFEA consultation on single embryo transfer closes this week. This new work raises concerns about the possible selection of single later stage embryos for transfer as this might increase the risk of identical twins.
The outcome for identical twins is worse than that for non-identical twins."
Dr Laurence Shaw, a fertility expert at the London Bridge Fertility Centre, said: "For the future, collapse rates could be used as a marker to improve our culture environments which will reduce twinning whilst at the same time improve pregnancy rates."