Image caption Many women choose to concentrate first on their career, leaving motherhood for later

Young women should freeze parts of their ovaries if they want to postpone motherhood until later in life, a US fertility expert has said.

Dr Sherman Silber told the American Society for Reproductive Medicine meeting in Denver a woman could freeze her ovary at 19 to use when she was 40.

Dr Silber, who says the procedure would work better than egg freezing, did the first full ovary transplant in 2007.

But UK experts warned ovary freezing had not been sufficiently tested.

Women are most fertile when they are young, with the chances to become pregnant diminishing with age, he said.

Although egg-freezing techniques are currently available at clinics in the UK, they usually harbour only a handful of eggs at a time.

It is far from enough to guarantee that a woman would be able to conceive when she decides to re-implant them in future.

Storing a part of an ovary may yield as many as 60,000 eggs, Dr Silber, who is based at the St Luke's clinic in Saint Louis, said.

He added: "The question is, how many cycles of egg retrieval do you need to feel comfortable and secure that you have enough eggs?"

"There's no absolute answer. Women who do egg freezing can't just have one cycle and think they've got it all solved."

And those who opt for several rounds of egg retrieval have to pay for every single procedure, making it "prohibitively expensive", he added.

But he said that removing and then freezing around a section of the ovarian tissue meant "one procedure and the whole thing is done".

'Too early'

Although there are already seven centres around the world that offer the storage of frozen ovarian tissue, there are none in the UK.

Tony Rutherford, chairman of the British Fertility Society, said the research was still very recent and much more needed to be done to ensure the procedure's effectiveness, especially in how successful the re-grafting of the ovarian tissue when a woman was ready to try and conceive would be.

"We don't know how many people have grafts and therefore we don't know how many have been successful and how many have failed," he said.

People need to know that it is not a guarantee Susan Seenan , Infertility Network UK

"We need to see clear evidence of [the method's] effectiveness and that's what we don't have at the moment."

Dr Silber claimed his hospital in Saint Louis has so far managed to carry out three successful ovarian tissue transplants using frozen tissue, which resulted in three births.

There have been 23 babies born from ovary or ovarian tissue transplants worldwide.

Fertility 'epidemic'

Dr Silber added: "We are in the middle of a fertility epidemic across the developed world and the reason our society is changing," he said.

Image caption Clinics that offer the storage of frozen eggs are available across the UK

"People are not trying to have children or are not even thinking about getting married until they are 35."

But Susan Seenan of Infertility Network UK told BBC News there was no guarantee that any of the currently used methods, including IVF, would ensure a woman could conceive.

"People should be very careful not to rely on this as a guarantee of success in the future, because you can't actually ensure your fertility.

"There could be other fertility issues, such as blocked tubes or an infertile partner.

"People need to know that it is not a guarantee."