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The first batch of frozen embryos, sperm and eggs given a 10-year time limit is due to expire in November, specialists say.
The batch was taken from 1650 people, many of whom may not know it could soon be destroyed.
In 2004, new legislation put a time limit on how long embryos and gametes (such as human sperm and ova) could be kept in frozen storage.
The material faces three outcomes: some will be disposed of, some donated to people wanting children, and the remainder given extended time limits in storage.
Fertility Associates medical director Dr Sarah Wakeman said about 220 people had applied to the Ethics Committee for extensions. Couples with frozen embryos wanting to complete their family were given shorter extensions but could apply for another on expiry, she said.
Many men froze sperm before having chemotherapy or radiotherapy and had forgotten about the stored matter, she said.
''Many have gone on to have children and have never needed it.''
Some men had been granted 20-year extensions because they froze sperm in their teenage years and had no immediate plans to start a family.
If an owner of frozen embryos or sperm could not be contacted, they would be disposed of, she said.
Contacting people could be difficult because a storage facility could not contact the owners' doctor, due to privacy laws.
Anyone wanting an extension should contact their fertility clinic immediately, she said.
Some couples whose IVF was successful chose to keep remaining embryos frozen due to their ethical beliefs.
To avoid having to decide on the fate of the embryos, many couples paid the $252 annual storage fee, she said.
But the new law would force a decision.
''It has prompted a few more embryo donations,'' Dr Wakeman said.
Some of the protocols for embryo donation were similar to those for adoption.
The couple donating the embryo would meet the recipients, who would also undergo counselling and police checks.
''All is designed to be in the best interests of any resulting child.''
However, the number of babies born from embryo donation was small and often an unappealing option for childless couples, she said.
The resulting child would be a ''full genetic sibling'' of children of another family, raised by people with no genetic relationship.
Legislated time limit
The Human Assisted Reproductive Technology Act 2004 was introduced because indefinite storage of embryos and gametes, such as sperm and eggs, raised many legal and ethical issues, a Ministry of Justice spokesman said.
The legal issues included uncertainty, or legal disputes, about what to do with stored gametes or embryos when the donor died and the potential for disputes about stored embryos or gametes when a relationship broke up.
The ethical issues included the potential for problems with an ever-growing pool of embryos, many of which would never be used. A statutory storage limit forced people to make decisions.
If there was no time limit, there was a greater possibility of people being born from human reproductive material by people who were deceased.
A 10-year limit allowed most couples time to realise their reproductive life plans. If the time was insufficient, an extension could be applied for, the spokesman said.
Australia had a five-year statutory limit on storage and Spain for the life of the donor.
The 10-year limit set out in the Act came into force on November 22, 2004, or from when storage of human reproductive material began, whichever was later, he said.