The next day, each egg is evaluated for evidence of fertilization. It takes about 18 hours for the egg to be fertilized. The first sign of fertilization is the presence of two small dots inside the egg called pronuclei- one from the sperm and one from the egg. The fertilized egg is called a zygote. The eggs that have fertilized are allowed to develop for two or more additional days.

About 12 hours later, the fertilized eggs start to divide into two cells, and subsequently into four and so on. By day 3 the embryos have cleaved (divided) into 6-10 cells and ready for replacement into the woman's uterus.

The transfer of several embryos increases the probability of success. A multiple embryo transfer also increases the risk of a multiple pregnancy. Embryos are transferred on either day three or day five of development.

With 2 - 3 good embryos on day 3 we advise a day 3 transfer as the pregnancy rate between day 3 and day 5 transfer remains the same. 40% of patients will not grow blastocysts 10% of the embryos that fail to develop to blastocysts in vitro may have done so if replaced inside the womb on day 3

After 5-6 days the embryo advances in development and becomes a blastocyst. Blastocyst transfer is claimed to be more physiological than cleaved-embryo transfer as it mimics nature more closely.

The improved implantation rates following blastocyst transfer is due to selection of the best embryos as it gives a better idea of the competence of an embryo.

There should be atleast 4 good embryos on day 3 to benefit from this procedure.

Blastocyst culture is indicated for : Patients who had repeatedly failed to achieve a pregnancy following the transfer of good quality cleaved embryos (If the growth of the embryos arrests and they do not develop to blastocysts, this may indicate a potential egg problem) Patients who wish to achieve a pregnancy without the risk of high order multiple pregnancy, Transfer of one or two blastocysts avoids multiple pregnancies