In some circumstances, an extended period of culture (4 - 7 days) can help in selection of the best embryo for transfer. A blastocyst stage embryo provides a somewhat greater likelihood of pregnancy compared with, for example, a transfer of a 4-cell stage embryo. A good functional method for freezing human blastocyst has not been develpoped yet, which affects the treatment options. It is worth noting that only approximately 4 - 6 embryos out of ten reach the morula stage when cultured. When there is a small number of embryos, this method cannot yet be recommended as the first treatment option.

In the case where the transparent surface layer surrounding the embryo (zona pellucida) is found in microscopic inspection to be noticeably thicker and harder than normal, it is possible to help the embryos to hatch with special techniques; by thinning or making a hole in the zona. This type of thicker structure of the zona may be the cause of infertility for some couples in that the normal implantation of the embryos into the uterus has been disturbed or obstructed.

A small number of couples may have singular genetic or chromosome errors which cause infertility. These errors may be apparent both in male and female gametes. Some of these errors may be detected by examining the family medical history, but some are newly generated and are discovered only when the couple in question is thoroughly examined. In some cases, analyses of blood samples by both partners can give information on the genetic background of infertility; in some

cases, singular gene tests can be done in order to determine the causes of infertility. All gene and cromosome errors do not, however, mean that the situation would be hopeless with respect to the embryos: one only needs to find a healthy embryo for the transfer!

A method called embryo biopsy followed by genetic analysis of the embryo (PGD, preimplantation genetic diagnosis) has been developed to solve this problem. Normally one or two biopsied cells from an 8-cell stage embryo are analysed for suspected gene or chromosome errors. After the analysis those one or two embryos that prove to be normal with respect to the error in question are selected for the transfer. This procedure can also be done later by amniocentesis or placental villus biopsy, but at this stage the fetus is already implanted in the uterus. The embryo biopsy techique is in use in our clinic, but its use requires a specific, real need. It is not possible to choose the sex of the embryo based on the wishes of the parents only, for instance.