The number of blunders made at IVF clinics has nearly doubled in the past 12 months.
The serious mistakes, which affect couples desperate for children, include cases where embryos have been lost or placed in the wrong woman, or incidents where eggs have been fertilised with the wrong sperm.
Figures released by the IVF watchdog reveal the number of reported incidents increased from 182 in 2007-08 to 334 in 2008-09, prompting calls for it to get tougher on failing units.
The figures do not show which fertility clinics were the worst-performing, but include centres throughout England and Wales where 50,000 IVF procedures took place in the past year.
A leading embryologist said the Human Fertilisation and Embryology Authority (HFEA) was not adequately enforcing the rules on fertility treatment.
Sammy Lee, from University College London, said: 'I think the key failure of the HFEA is that when they ask clinics to put in special procedures, they're not enforcing them.
'It's important that when you've identified a weakness in a procedure, you quickly enforce it, and don't wait a year to do so.'
The figures include all reported incidents and near misses to the HFEA during 2008-09.
Examples include fridges containing eggs being switched off accidentally, or consent forms not being signed by two clinicians, which is against the rules.
The blunders are revealed on Donal MacIntyre's BBC Radio 5 Live show tonight.
The programme features an interview with one woman, identified only as Clare, who was told by clinicians at the University of Wales IVF clinic in Cardiff that two of her remaining three embryos created during her first cycle of fertility treatment had been 'lost'.
She says the only explanation staff could offer was that they may have 'slipped off the straw' during the freezing process.
Clare, who had been trying for a baby for seven years with her partner Gareth before beginning treatment at the clinic in 2008, said: 'I was waiting to go in and have a transfer and they said I only had one embryo remaining - the other two had gone missing. Those were two potential babies.'
The clinic said it would not comment on individual cases but its overall rate of frozen embryo recovery was 'high' by international standards.
Lawyer Guy Forster, who is representing Clare and who has dealt with similar cases in the UK in the past year, called the situation 'deeply disturbing'.
The HFEA said it did not accept it needed reform. A spokesman said: 'In embryology, as in all areas of clinical care, it is not possible to guarantee 100 per cent success.'
This site contains copyrighted material the use of which has not always
been specifically authorized by the copyright owner. We are making such
material available in our efforts to advance understanding of
biotechnology and public policy issues. We believe this constitutes a
'fair use' of any such copyrighted material as provided for in section
107 of the US Copyright Law. In accordance with Title 17 U.S.C. Section
107, the material on this site is distributed without profit to those
who have expressed a prior interest in receiving the included
information for research and educational purposes. For more information
go to: http://www.copyright.gov/title17/92chap1.html#107. If you wish to use
copyrighted material from this site for purposes of your own that go
beyond 'fair use', you must obtain permission from the copyright owner.