Introduction

Preimplantation genetic diagnosis (PGD) is a procedure developed to help couples who are at risk of transmitting an inherited disease to have a healthy family. Apart from PGD, the main option for these couples is prenatal diagnosis (amniocentesis or chorionic villous sampling) between 10 and 16 weeks' gestation to determine if the fetus is affected. If prenatal diagnosis shows an affected fetus, the couple must then decide whether to continue with the pregnancy or undergo a termination. Other options are natural conception with the associated risk of having an affected child, gamete donation or to remain childless.

Couples referred for PGD are known to be at genetic risk due to an affected family member or the birth of an affected child and may:

• have a previous history of prenatal diagnosis and termination of an affected pregnancy;

• have moral or religious objections to termination of pregnancy;

• have a previous history of recurrent spontaneous miscarriages due to chromosomal abnormalities;

• be infertile and carrying a genetic abnormality (which may or may not be causing their infertility).

PGD involves three stages: IVF, embryo biopsy and single cell diagnosis. Although many PGD couples are fertile, routine IVF procedures are required so that embryos are generated outside the body for biopsy and PGD and unaffected embryos can be subsequently transferred to the woman. It is possible to remove cells from the embryo at three stages (Table 12.1), but the most common method is cleavage stage biopsy. Polymerase chain reaction (PCR) or fluorescent in situ hybridization (FISH) are used for single cell diagnosis (Table 12.2).