Comprehensive Chromosome Screening (CCS) / Preimplantation Genetic Screening (PGS) and Preimplantation Genetic Diagnosis (PGD)

What information can be obtained from biopsying (removing cells) an embryo?

At different developmental stages, an embryo may have cells removed (biopsied) to gather information about its health. At CCRM OC Fertility we typically biopsy embryos at the blastocyst stage (day 5-7 after egg fertilization) to improve test accuracy, since more than one cell may be safely removed at this stage and embryos that make it to a blastocyst have already proven a survival advantage.

The few cells that are removed may be analyzed in different ways to gain knowledge about the health of the embryo (and potential fetus and resultant child). For instance, the analysis may reveal whether or not the embryo has the correct number of chromosomes, 23 pairs – one pair from mom and one pair from dad. Problems that result in too many or too few chromosomes lead to abnormalities in offspring, such as Down’s syndrome (Trisomy 21, three copies of Chromosome 21). This risk of abnormal chromosome number to the embryo (called aneuploidy) increases as women age, especially as a woman enters her late 30s and 40s.

Two key technologies that require obtaing cell DNA by biopsying an embryo are Comprehensive chromosome screening (CCS) / Preimplantation genetic screening (PGS) and Preimplantation Genetic Diagnosis.

What is Comprehensive Chromosome Screening (CCS)/Preimplantation Genetic Screening (PGS)?

CCS/PGS allows us to analyze biopsied cells from an embryo to determine whether there are chromosomal errors present before implantation. This is done to avoid implanting embryos that will not lead to a healthy pregnancy, or often will result in miscarriage or diagnosis later in pregnancy. The types of chromosome errors increase with frequency and complexity as a woman ages. This explains the lower pregnancy rates and higher miscarriages seen among older women.

While all men and women have a portion of their eggs and sperm that are chromosomally abnormal, the likelihood of chromosomal errors increases due to age and the health history of the parents, particularly the mother’s age.

Using this technique, we often uncover the cause of “unexplained infertility” in younger women, those under 35 years of age.

When Is It Used?

CCS/PGS is used to treat: infertility due to advanced maternal age (older than 35 years), recurrent miscarriage, poor ovarian function in younger women, embryo banking in family building, gender selection, or multiple failed fertility treatments. Some may also choose to have CCS/PGS for personal reasons – to limit the chance of having to decide to terminate a pregnancy later in pregnancy because of a diagnosis of a chromosomal problem. Finally, some patients opt to do CCS to maintain high pregnancy rates while limiting the risk of multiple gestation pregnancies by transferring only one embryo at a time.

What Is Preimplantation Genetic Diagnosis (PGD)?

In addition to testing for chromosome errors, the cells may be analyzed for other known familial hereditary conditions or gene mutations. Examples include cystic fibrosis or muscular dystrophy. This technology is referred to as Preimplantation genetic diagnosis (PGD).

When Is It Used?

In contrast to CCS/PGS, PGD may be recommended for couples who have or are carriers of a known genetic disorder that may lead to disability or disease in their children, such as cystic fibrosis or muscular dystrophy; as well as balanced translocations. Not only can this treatment identify unaffected, but possible carrier embryos, but it may allow a family to eliminate the risk of transmitting a specific mutation to future generations by identifying unaffected non-carrier embryos.

What Do I Need to Know?

CCS/PGS and PGD are elective techniques performed as part of the IVF process. Several cells are taken from an embryo (at CCRM OC Fertility. we biopsy embryos at the blastocyst stage) and sent for genetic testing while the embryo is frozen to wait for the biopsy results. Depending on each case, a couple may have an indication to have the cells tested for both chromosomes (CCS/PGS) and specific gene mutations or translocations (PGD). Whether or not you are a candidate can be discussed with your physician.

Information Source: American Society for Reproductive Medicine