Applications of Preimplantation genetic screening (PGS) and Preimplantation genetic diagnosis (PGD) for achieving pregnancy
Preimplantation genetic screening (PGS) In this procedure, embryos, which are created through In Vitro Fertilization (IVF) are checked for chromosomal abnormalities before transferring them into a uterus. Most of the miscarriages occur in the embryos carrying chromosomal abnormalities.
To enhance the chances of a successful pregnancy, PGS is sometimes offered to people with recurrent pregnancy loss or infertility. For screening of embryos with an abnormal number of chromosomes preimplantation genetic screening (PGS) is used. If an embryo contains abnormality in the number of chromosomes, it may lead to miscarriage, Edward’s syndrome, or even Down’s syndrome. To improve the chances of a successful pregnancy and a healthy child, embryos that are found to have a genetic abnormality will not be used for transfer to the uterus.
Preimplantation genetic screening (PGS) and preimplantation genetic diagnosis (PGD) are two different preimplantation tests. Embryos with unusual numbers of chromosomes are identified in PGS, but to identify embryos that carry a single-gene disorder, PGD is employed to identify the risk of mutation in a specific gene.
The main difference between PGS and PGD lies in the fact that PGS is used for screening out chromosomally abnormal embryos, whereas and PGD is used for diagnosing specific defects that a fetus is likely to inherit. PGD is offered to the patients with Cystic Fibrosis or those who have another biological child with a genetic disorder or families with the history of severe genetic disorders. Both PGS and PGD can be performed at the same time if required.
A sample of cells is collected and karyotyped from the embryo in PGS. This is dependent on the developmental stage of an embryo. In 3- or 5-days embryos can be biopsied for examination.
An embryo reaches the blastocyst stage of development on the 5th day. A blastocyst contains 200 cells approximately, so removing a few of them are considered to be less destructive to the developing embryo than at the earlier stages of development.
Before implantation, cells are removed from the blastocyst using a trophectoderm biopsy and screened for genetic abnormalities. The couples at increased risk of chromosome abnormalities or specific genetic disorders are benefited from PGS and PGD. Embryos without any abnormalities are transferred to the uterus to achieve pregnancy.
The advantages of PGS are associated with the maternal age; PGS mitigates several reproductive challenges, offers greater implantation rates, and improves IVF outcomes. It also reduces the chances of multiple births and complications as it enables single embryo transfer with higher success rates.
Through preimplantation genetic screening and morphological assessment, 69.1% pregnancy rate can be achieved, which is much higher than the 41.7% achieved by using only morphological screening.