By Caroline Ryan
BBC News, Prague
A new egg-freezing technique could give women a better chance of having a baby when they are older, say scientists.
Freezing can damage eggs
Freezing and thawing eggs has carried a high risk of damage, and of 153 treatment cycles in the UK between 1999 and 2002, just one baby was born.
The new Japanese-developed technique offers a ten times higher chance of a successful pregnancy.
Using an antifreeze method, it has led to 11 babies being born, a European fertility conference was told.
The new technique offers hope to women whose fertility may have been damaged by cancer - and those who decide to put having a famly on hold.
So far 50 career women from around the world have chosen to use it to store their eggs.
Although sperm can be successfully frozen, thawed and used in IVF, helping thousands of couples, it has been very difficult to do the same with eggs.
Sperm is the simplest cell in the body, and is easy to collect, while a human egg is the most complex cell - and women only produce one per cycle.
Old methods involved simply freezing eggs.
Dr Masashige Kuwayama, scientific director of the Kato Ladies Clinic in Tokyo, Japan, told the Prague conference about his Cryotop method, first developed by the team for artificial insemination of sheep and cattle.
It uses a kind of antifreeze, which removes water from the eggs and prevents damaging ice crystals forming.
Other similar techniques have been tried, but this freezes eggs much faster, and uses less solution, reducing the risk of contamination.
Using the technique, Dr Kuwayama froze 111 eggs, of which 94% survived the freezing-thawing process.
Twenty-nine embryos were created and successfully transferred into women, resulting in 11 babies being born.
Existing techniques result in one baby per 100 eggs, while the new method will lead to 10 babies being born per 100 eggs used, researchers said.
Dr Kuwayama said: "This technology opens up new horizons for medically assisted reproduction in women, enabling them to have the option of having children at a later date by freezing eggs rather than embryos.
"Moreover, it will help to eliminate the existing time differences in fertility between men and women, whereby women's supplies of eggs decline at a faster rate than men's supplies of sperm."
He said the technique could be helped to use any woman who chose to freeze her eggs.
Natural conception 'best'
Professor Arne Sunde is a former chairman of the European Society for Human Reproduction and Embryology (ESHRE).
He said: "For decades men have had the opportunity to freeze sperm prior to treatment for malignant diseases, and thousand of babies have been born to couples where the male is an infertile survivor of cancer treatment.
"With the current techniques, women would need to freeze hundreds of eggs in order to have a reasonable chance of obtaining a child.
"But by using this technique, it will be possible to achieve the same rates of success with 10-20 frozen-thawed eggs as with fresh eggs.
"This is a major improvement, and for the first time, cryopreservation of oocytes [egg freezing] represents a realistic option for the preservation of fertility in women who are in need of aggressive treatment for malignant diseases."
But he said women hoping to secure their chances of having a baby after they have had a career should not rely on this technique.
"Women should try to conceive early and naturally.
"If they do not succeed, they should come to us."
John Paul Maytum, a spokesman for the Human Fertilisation and Embryology Authority, said: "Our independent panel of scientific experts have looked at this technique and have no fundamental concerns but believe that there should be proper training and protocols if it is being carried out."