The legal limbo that prevents Natallie Evans from using the embryos she created with her former partner has its origins in a lack of options for women who want to preserve their fertility before cancer treatment.

While men have long been able to bank sperm before having therapy that could leave them sterile, women have been denied similar opportunities because of the difficulty of preserving their eggs for future use.

Both sperm and embryos can be frozen and thawed with high success rates, but eggs have proved much more resilient to being kept on ice.

Eggs are much larger than sperm, many fewer of them are produced, they are considerably more fragile and they contain more water. When they are frozen this water forms ice crystals that readily damage the egg when it is thawed.

Although the first baby conceived using a frozen egg was born in 1986, only about 200 more have been born worldwide since.

This has forced women like Ms Evans to have eggs removed and fertilised while still fresh, to create embryos that can be stored in liquid nitrogen.

This is not usually a problem for women in stable relationships, but always carries a risk that her partner will withdraw his consent for the embryos’ use at a later date.

It is also a poor option for women who do not have a partner with whom they wish to have children, who must have their eggs fertilised by a sperm donor if they wish to store embryos. This technique is also useless for girls who are too young to produce mature eggs.

New technology, however, is now starting to solve this problem, though it has come too late to help Ms Evans, who has already had both ovaries removed.

Scientists have recently developed much more efficient techniques for freezing eggs, which can now be thawed without the formation of ice crystals. A procedure called vitrification, in which moisture is removed from the egg and antifreeze added before it is frozen, allows 95 per cent of eggs to survive thawing and leads to IVF pregnancy rates of about 25 per cent.

There are now 23 British clinics licensed to freeze eggs, although only two British women, Helen Perry and Margaret McNamee, have given birth using frozen eggs. Both chose the procedure because they had religious objections to freezing IVF embryos that might never be used.

A second new technique, by which ovarian tissue is removed and frozen before women have cancer treatment, is also considered highly promising. Two women, one in Belgium and one in Israel, have had healthy babies after slices of their ovaries were removed, frozen and then re-implanted after successful cancer treatment.

Many doctors believe that the procedure will eventually be a better option than egg freezing, as ovarian tissue survives freezing well, restores menstrual cycles when it is re-implanted, and contains many more eggs than it is usually possible to collect before a woman has chemotherapy or radiotherapy.

It also has the potential to help pre-pubescent girls to preserve their fertility, as scientists believe that it will be possible to ripen the immature eggs in ovary tissue, either by re-implanting it or in vitro.

"Women don’t have the luxury of preserving their fertility in the way that men do prior to cancer treatment," said Allan Pacey, of the University of Sheffield. "Once we perfect the technology to freeze eggs or ovarian tissue as a matter of course for women, this sort of situation will be avoided."

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