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Too many to cite -- I read everything I see on this topic. But there are three main routes, all addressing the fact that neither womb age (including post-menopause) nor egg nucleus age poses any significant obstacle to successful pregnancies in late middle age or beyond; the main obstacle is the age of the non-nucleus part of the egg (cytoplasm), which is not the part which determines genetic parenthood.

One method is replacing the nucleus of a donor egg with a nucleus from an egg from the genetic would-be mother -- this was done succesfully for a short time before the Luddites succeeded in getting it stopped by screaming "but that's CLONING" (in the U.S. -- it is still being done elsewhere). It will resume here in due course. A variation on this procedure, which as far as I know has not been stopped, involves tranfer of some cytoplasm from the donor egg into the would be mother's egg (called "cytoplasmic transfer"). This method does not reduce the problem of genetic abnormalities which are more common in older women, but through pre-implantation genetic diagnosis, occurrences of most of those can be identified and not implanted.

The other method (already being used experimentally on women who have under cancer treatment) is removal of all or part of an ovary, freezing it, and replacing it in the body years later. The ovary then begins behaving as if it was the age that it was when it was removed and frozen. I don't believe any serious attempts at human pregnancy have been made with this method so far, but it has restored perfectly normal menstrual periods to women who had had both ovaries removed either due to the ovaries being diseased (cysts), or to prevent damage from various treatments for cancer in other parts of the body. There is every indication that it can produce normal pregnancies as well, most likely without the need for in vitro procedures. This method does eliminate the problem of age-related genetic abnormalities.

There is one fertility outfit that claims to have developed a new procedure for freezing and thawing unfertilized eggs (an approach which has had little success so far). Their claims have not been peer-reviewed as yet, because they have been unwilling to disclose the specifics of the method. If this type of approach can be made reliable, it will be ideal, as it will allow relatively simple extraction and preservation of unfertilized eggs, for use years later, without the need to choose the father in advance.

For women who marry relatively young, but want to postpone childbearing, freezing embryos is already a reliable method of buying quite a few years.

For all but the first of these methods, the important thing is to plan ahead. But I expect that will become common practice, as these procedures become commercialized and widely used.

I don't doubt for a moment that science and technology have extended the length of the clock, and I don't have a problem with this. Medicine has allowed us to live longer than we could 100 years ago, and this is unquestionably a good thing for mankind.

But there is more to the biological clock than just the number of eggs a woman has at birth. Waiting longer until pregnancy increases the physical danger to the woman as well as the child. Not to mention that physically, hormonally, and emotionally, a woman in her thirties is far more naturally suited to the difficult task of raising and caring for an infant than a woman in her fifties. Of course a wealthy woman could always just decide to have nannies do all the hard work, but I don't think this is a trend that we want to encourage. Heck, a sperm donor and a turkey baster even make the need for a male partner biologically redundant, but I believe men are far more than just mere sperm donors.

Bottom line is, the clock can and will be extended, but it still exists, and it's a bit of a stretch to say that today's women won't have to worry about it a bit.