Preimplantation genetic diagnosis (PGD) is an early form of prenatal diagnosis, developed in the late 1980s to help couples who are at risk of transmitting an inherited disease to their offspring. If such couples wish to have a healthy family, the only option open to them apart from PGD is prenatal diagnosis by amniocentesis or chorionic villus sampling (CVS). The main disadvantage of prenatal diagnosis is that if the diagnosis shows the fetus to be affected, the couple have to decide whether they wish to terminate the pregnancy or carry on with the knowledge that their child is going to be affected by the genetic disease. PGD offers some of these couples an alternative, as the diagnosis is performed on the preimplantation embryo and only unaffected embryos are transferred to the patient. The pregnancy is therefore initiated with the knowledge that the fetus is free from the disease.

When PGD was first developed, the main referral group consisted of patients who had already experienced several terminations of affected pregnancies, or those with moral or religious objections to termination. However, in recent years PGD has been used to help patients who have experienced repeated miscarriages due to unbalanced chromosome arrangements in the fetus. In addition, some infertility patients, mainly those treated for severe male infertility by intracytoplasmic sperm injection (ICSI), have been found to carry chromosomal translocations; PGD can be performed for these couples so that normal embryos are replaced.