Traditionally, embryo transfer following IVF or ICSI takes place two or three days after fertilisation, when the embryo contains around six to eight cells. Once fertilised, the eggs are left to develop as embryos in culture medium in an incubator, and throughout this time they are checked for growth, shape and composition.
However, recent results from IVF clinics around the world suggest that pregnancy rates will be improved if the embryos are left in the incubator to develop to their blastocyst stage - that is, when they are five or six days old. A blastocyst has many more cells than an embryo and, because of its longer development, may well be more viable for transfer and pregnancy. Indeed, in a natural pregnancy it is a blastocyst, not a three-day-old embryo, which implants in the uterus.
Despite the good results, not all patients are suitable for blastocyst transfer, especially those with only one or two good quality embryos at day two or three. Not all embryos will develop into good blastocysts, so it is often best to avoid any risks and go ahead with transfer with day two or three embryos. Embryo assessment represents an important decision for the embryologist, and will largely determine whether blastocyst transfer is suitable or not.
The most common reason for an IVF or ICSI cycle to fail is because embryos fail to implant. Before an embryo can implant into the lining of the uterus it must 'hatch' out of its 'shell'. This usually occurs five or six days following fertilisation.
There are many reasons why successful implantation does not occur. One of these reasons may be due to the fact that the embryo is unable to 'hatch' because the 'shell' (zona pellucida) is too thick, or too hard. Assisted hatching is a laboratory procedure whereby a hole is made in the 'shell' of a two or three-day old embryo in order to help in the 'hatching' process and therefore, help with the implantation of the embryo into the endometrium. This two or three day old embryo is then placed back into the womb.