The freezing and thawing of human embryos can double or even triple the number of chromosomes in living cells, with potentially serious consequences for unborn offspring.
The process was observed in just 1 in 30 instances, but researchers in Canada believe it possible that spontaneous cell fusion might be linked to problems associated with implanted "frozen" embryos.
Hanna Balakier, a scientist at the Success through Assisted Reproductive Technologies (Start) clinic in Toronto, said: "This is a signal to watch frozen embryos, see if such events occur and then exclude the affected embryos before implantation."
Balakier and colleagues at Start and the University of Toronto studied two and three-day-old human embryos in vitro . Their results are published in the journal Human Reproduction .
They found that some of the cells fused, forming a host of cells with more than one nucleus and, hence, more than the usual pair of each chromosome.
There is no indication that children born of frozen embryos suffer chromosomal abnormalities as a result. But more problems are encountered in pregnancy when frozen embryos are used rather than "fresh" ones.
Balakier said fertility clinics could screen early embryos after thawing to reject those with fused cells. But she said it would be much harder to do anything about embryos that were frozen at a later stage in development. As they contain so many more cells, flaws would be very difficult to spot.