Pre-implantation Genetic Screening (PGS) is a test to examine the chromosomal material of embryos developed through IVF to detect chromosomal abnormalities before the implantation. In contrast to PGD where the test is done specifically when one or both parents has a known genetic abnormality pre-implantation genetic screening (PGS) is done to embryos from presumed chromosomally normal genetic parents which are screened for aneuploidy( abnormality in the number of chromosomes) PGS is not a test for specific diseases
PGS involves removing one or more cells from the embryos developed through IVF for numerical abnormalities in the chromosomes (Aneuploidy). This test helps the implantation of embryos having the normal number of chromosomes (Euploid embryos). Normal healthy humans have 23 pairs of chromosomes, this test also known as Genesis-24 examines all the 24 chromosomes at the time of the embryo transfer of which 22pairs are the non-sex chromosomes and 2 are the sex chromosomes X and Y.
Chromosomal abnormalities may be in the number (aneuploidy) as in Down’s syndrome,or translocations where the chromosomes are not incorrect positions, or structural alterations that may be clinically significant.
PGS is mainly prescribed for couples of advanced maternal age, known familial translocations or inversions, gene duplications, and deletions, or families who have had recurrent pregnancy losses both naturally and through IVF. The screening aims at improving the pregnancy rate by testing the embryos for chromosome abnormalities—an increasing problem for families who choose to have children later in life. The embryos are chosen based on the visual quality and morphology if PGS screening is not done. In cycles that incorporate PGS, embryos are further assessed based on their chromosome complement. Embryos with the normal number of chromosomes are more likely to result in a pregnancy that is carried to term. Since these tests are not an essential step of the IVF process, the cost of PGD/PGS is usually not included in the cost of an IVF cycle.