PGD is a procedure in which one of the cells (blastomere) of the embryo is removed for genetic analysis three days after egg retrieval to identify genetically abnormal embryos. By not transferring abnormal embryos, this procedure may reduce the chance of miscarriage, or when done to identify specific genetic disorders, may prevent the need for termination of pregnancy or the birth of a baby with a serious genetic disease.
A wide variety of genetic disorders can be detected by PGD. However, this is only appropriate in couples known to be at increased risk for a genetically abnormal offspring. One example is when both partners are carriers for the cystic fibrosis gene. One fourth of their embryos would carry both abnormal genes and would be affected with this serious disease. One half of the offspring would be carriers, like the parents. With other types of genetic diseases on average half of the embryos will be abnormal. In some sex-linked disorders, like hemophilia, only the male offspring will usually be affected; females may be carriers, but rarely affected.
In other instances, the couple may be at increased risk for miscarriage because of one of the partners carrying an abnormally located chromosome (translocation carrier), age or because of repeated miscarriage. PGD may reduce the risk of fetal loss in such couples.
The most common indication for PGD is when the female is older (over 35 or particularly over age 40). The chance of successful pregnancy steadily falls and the chance of miscarriage steadily rises with increasing age because more of the embryos are genetically abnormal. Such couples can choose PGD so that only normal embryos are replaced. The nine chromosomes that are most frequently abnormal are analyzed, so not all genetic abnormalities may be detected. Although the risk of miscarriage is reduced, it still can occur.
Although the chance of finding a genetically abnormal fetus is lower than in cases when PGD has not been performed, prenatal diagnosis by amniocentesis or CVS is still recommended since PGD does not detect all genetically abnormal embryos. If there are at least six fertilized eggs, reports have indicated that the chance of pregnancy may be increased, although some additional pregnancies might have occurred with frozen embryos in cycles when PGD was not performed. To put it another way, couples with many fertilized eggs may have an increased chance of pregnancy with the fresh embryo transfer because more normal embryos are replaced rather than being frozen, thus increasing the efficiency and perhaps the overall success resulting from that egg retrieval.
Couples with fewer than six fertilized eggs do not have an increased chance of pregnancy with PGD, but the risk of miscarriage is reduced. Since about 5 10 % of embryos may be called abnormal when other cells of the embryo are normal, and since there is some small impact of removing a cell from the embryo, we do not advise PGD for couples with fewer than six fertilized eggs unless the reduced rate of miscarriage is the primary goal. Finally, there are instances when the test may be indeterminate in which case the particular embryo could be normal or abnormal. If there are not enough other normal embryos for transfer, the couple may elect to transfer such an embryo.
Embryo transfer is done five days after egg retrieval (blastocyst transfer). Couples will sign consents for PGD for RPMG and for the PGD laboratory, and for blastocyst culture. Because it requires highly sophisticated micromanipulation techniques and transport of the cells to a very experienced laboratory for an expensive analysis, PGD adds significant cost to the IVF cycle.