A woman has given birth to a baby boy who was conceived using 21-year-old sperm, it has emerged.
The boy was conceived after four attempts at IVF
Doctors at St Mary's Hospital in Manchester, where the baby was born two years ago, said the age of the sperm might make it a world record.
The boy's father had his sperm frozen when he was 17 before undergoing treatment for testicular cancer, which made him infertile.
Doctors said the case showed that freezing sperm was safe and worthwhile.
Details of the case were published in the journal Human Reproduction.
The man's sperm was frozen until he got married and wanted to start a family.
It was thawed and then injected into his partner's eggs. The baby was conceived after their fourth attempt at IVF.
"This case report provides evidence that long-term freezing can successfully preserve sperm quality and fertility," said Greg Horne, a senior embryologist at the hospital.
"This is important to know because semen stored by young cancer patients is undertaken at a time of great emotional stress when future fertility is unlikely to be an immediate priority."
The doctors said it was probably a world record.
"We believe this is the longest period of sperm cryopreservation resulting in a live birth so far reported in the scientific literature," said Dr Elizabeth Pease, a consultant in reproductive medicine at the hospital.
However, she said new limits on the number of free IVF treatments would have meant that the couple would not have been able to have their baby on the NHS.
Dr Virginia Bolton, a consultant embryologist at King's College Hospital in
London, welcomed the news.
"Certainly for people who have had their sperm frozen very young because of having to have cancer treatment this is obviously very welcome news.
"However, it is not hugely surprising. From animal studies the only damage it
seems that could occur to frozen sperm is through background radiation."
But she added: "It is important because we can feel reassured that it's not going
to be a problem and it gives hope to young people who may want to have children
a long time after their cancer treatment."
Under UK regulations, frozen sperm can be stored until a man is 55.