A hospital has moved to reassure couples whose IVF embryos were accidently mixed with sperm from other men.
The hospital said the error was an "isolated incident"
Liverpool Women's Hospital admitted a group of embryos had been exposed to sperm from other men during the summer.
The error, which involved embryos belonging to 10 couples, was discovered when a member of the hospital's reproductive medicine unit detected possible "contamination" in the liquid in which they were being stored.
But consultant gynaecologist, Charles Kingland, said the embryos were not at risk - although only one of them managed to implant and go on to produce a viable pregnancy.
The addition of the sperm posed no risk of changing the genetic makeup of the embryo, the hospital insisted.
Each of the couples involved and the Human Fertilisation and Embryology Authority (HFEA) - which regulates IVF clinics in the UK - were informed.
The HFEA said it was satisfied with the hospital's procedures.
However, the IVF cycles involving the embryos continued, and they were implanted into the women, prior to the mistake being discovered.
Of the 10 embryos, eight did not result in pregnancy and of the remaining two, one miscarried.
Mr Kingland said that there was no question that the mistake could influence who "fathered" the embryo - as fertilisation had already taken place and could not be reversed.
He said: "It can't happen - end of story."
A spokesman for Liverpool Women's Hospital NHS Trust said all treatments were immediately discontinued after the error was discovered.
She added: "Following a thorough investigation, the risks to each couple were quantified as being exceptionally low to negligible and the HFEA were satisfied with their findings.
"All couples were appropriately counselled and treatments then recommenced.
"This was an isolated incident which was immediately recognised and corrected and this has not affected any other patients."