At 2 days after fertilisation embryos are about 4 cells, after 3 days 8 cells (blastomeres) a blastocyst develops at around day 5.

There was an article in the Winter 98 edition of Issue by Dr Sharon Mortimer of the University of Sydney - this is a bit of what she says "In nature fertilisation occurs in the fallopian tube, or oviduct. After fertilisation the first cell divisions take place in the oviduct. After several days, the embryo has travelled all the way down the fallopian tube and entered the uterus (at about day 5 after fertilisation). At this point, the embryo is a hollow ball of cells, called a blastocyst. The blastocyst must now implant in the lining of the uterus. In routine IVF treatments, the embryo has only divided to the 4-8 cell stage when it is transferred back to the patient's uterus on days 2-3 after fertilisation. This means that the embryo must continue dividing to form a blastocyst inside the uterus, rather than inside the fallopian tube. It is not possible to predict which embryos will be able to continue to develop into blastocysts before the embryo transfer.

Until very recently it was not possible to keep embryos in culture for enough time for blastocysts to develop. This was because the culture conditions could not keep the embryo's development continuing at the same rate that it would occur inside the fallopian tubes, or even inside the uterus. The lining of the uterus, in which the blastocyst implants, also develops and changes and is only receptive to an embryo for a fairly short timespan. This means that if the embryo develops more slowly than the lining of the uterus, by the time the blastocyst is transferred back to the patient there won't be any chance of implantation.

...The research work has involved the development of new culture media, as well as using new technology to maintain the temperature and environment for embryos in the laboratory. It used to take six days for embryos to be able to develop into blastocysts in the laboratory, now it takes only 5 days - the same rate of development as inside the body. This means that if these blastocysts were transferred, we would expect that the lining of the uterus and the development stage of the embryo would be synchronised. A very recent clinical trial in America involving 23 patients has shown an increased proportion of embryos implanting following blastocyst transfer (45% per embryo transferred versus an expected implantation rate of 10-15% for conventional IVF) with an ongoing pregnancy rate of up to 60%. However, as with all new trials, it is important to be cautious with the first results and make sure they can be sustained with a larger group of patients before they can be considered to be conclusive."

From what I have read the US and Australia are a bit ahead of us in perfecting the culture media and I know that the Oxford (JR) were researching this area recently. Unfortunately not all fertilised eggs will become blastocysts so although you may start out with a lot of eggs you may only get a few if any blastocysts.

There's also an article is Spring 1999's iSSUE magazine by Simon Fishel. One of the aspects he covers is extended culture and blastocyst transfer. He basically says that, currently, there is no real scientific backing for going to blast transfer.

He cites various arguments including the quality of culture medium versus the natural environment in the uterus and the methods used by various clinics to report their success rates. One issue was that blastocyst transfer favours patients with a large number of embryos because, by the law of averages, they are likely to have more embryos surviving to blast than those with only two or three embryos. He also states that it doesn't necessarily follow that because an embryo didn't survive to blast stage that it also wouldn't have survived in the uterus if it had been transferred earlier.

His final paragraph reads : "Logically, one could take the view that if extended culture had definitely been proven to be beneficial, it would have been adopted by many more clinics than has been to date. I have little doubt that blastocyst transfer would be enormously beneficial providing we can improve our current in vitro culture conditions. However, I remain unconvinced of the data proffered to date, except that there seem to be many more patients failing to undergo embryo transfer."