The embryo(s) can be transferred to the uterus at the cleavage stage when the embryos are at the 4-8 cell stage (about day 2) or at the blastocyst stage (day 5-6) - often under ultrasound guidance to monitor the position where the embryos are placed.

The multiple pregnancy rate when more than one embryo is transferred is now felt to be too high putting unacceptable burdens on stretched neo-natal services with premature births, low birth rates and possible complications in later years.

For many years the culture methods were comparatively simple and its was felt that it was safer to transfer the embryos into the safe environment of the uterus as early as possible.

A better understanding of the changing requirements of the embryo as they develop has resulted in the availability of commercially available culture medium capable of growing the embryos to the blastocyst stage.

An added bonus is that acceptable pregnancy rates can be achieved with single embryo transfer, thus reducing the incidence of multiple births. Many people feel that transferring the embryos at day 5-6 is much closer to the arrival time of the embryo in the uterus during spontaneous conception - i.e. it is much closer to the way which has evolved.

Some embryologists also feel that they are able to judge the quality of the embryos better at the blastocyst stage than the cleavage stage and so can improve the chances that the embryo will implant.