Sperm can be frozen for ever

Diane Blood became pregnant by her dead husband using a technique known as intracyctoplasmic sperm injection (ICSI).

Sperm was extracted twice from her husband Stephen, 30, by doctors while he was on a life support machine after contracting meningitis in 1995.

The sperm was mixed with a solution called a cryo-protectant, which reduces the risk of damage during the freezing process.

It was then placed in test tubes, sealed and frozen in liquid nitrogen which keeps it in a state of "suspended animation" and means it can be shipped anywhere.

In Mrs Blood's case, she took the test tubes to Europe to circumvent the British ban on using her dead husband's sperm.

Dr Virginia Bolton, a clinical embryologist at King's College Hospital said: "Provided it is kept in this state, the sperm can, in theory, last for ever." The legal limit for keeping sperm in frozen storage is 10 years but this can be extended under certain conditions.

When Mrs Blood wanted to use the sperm it was slowly thawed, causing the cells to become active and mobile again. She was given hormone drugs to boost her egg production for a few months.

Eggs were then collected from her and put into a petri dish, where they were stripped of all outer cells.

The most mature, or ripe eggs, were chosen and using a microinjecting technique a single sperm was injected into the wall of each egg. It was then down to which eggs developed into embryos. After two to three days, a maximum of two fertilised embryos - the most healthy looking of the bunch - would have been implanted in Mrs Blood's womb.

The next 14 days would have been an agonising wait to see if the process had made her pregnant.

Dr Bolton said: "If you are 37 or under and can produce a number of good eggs, you have around a one in three chance of becoming pregnant.

"The ICSI technique is now used as often as IVF because the success rates have gone up in recent years."

The technique was introduced in Britain in 1994.