The difference between PGD and PGS in the third generation IVF technology

In recent years, China has gradually successfully applied PGD/PGS for in vitro fertilization. What are the differences between PGD and PGS technologies, and where are they used?

What is PGS/PGD?

PGS is a pre implantation genetic screening for embryos. Before embryo implantation, it mainly compares and analyzes the structure and number of 23 pairs of chromosomes in the embryo through NGS (high throughput sequencing technology) to determine whether there are genetic abnormalities in the embryo. PGS is used to detect chromosome number and structural abnormalities in early embryos, mainly to check whether the chromosome structure and number of the embryo are normal.

PGD is pre implantation genetic diagnosis of embryos, mainly using FISH technology (fluorescence in situ hybridization), which uses fluorescent labeled specific genes or chromosome fragments as probes to hybridize with chromosome specific sequence DNA molecules to determine the position and number of corresponding chromosome sequences in dividing or interphase cells, in order to detect chromosomal numbers or structural abnormalities. After sperm and egg combine to form a fertilized egg and develop into an embryo, PGD technology is used for genetic testing before implantation into the uterus to prevent some genetic diseases in IVF. At present, pre implantation genetic diagnosis can already diagnose some single gene genetic diseases, such as hereditary hearing loss, polycystic kidney disease, and so on.

What is the difference between PGS and PGD?

Both PGS and PGD are used to screen for the health status of pre transplant embryos, but the most significant difference is that PGS is a genetic screening, while PGD is a diagnosis.

PGS is a genetic screening that targets all chromosomes in an embryo, allowing for the detection of missing logarithms and normal morphological structures. PGS is screened after the formation of embryos from fertilized eggs (on day 3 of cultivation) or the formation of blastocysts (on day 5 of cultivation). Embryos with chromosomal problems are difficult to naturally develop to maturity, and it is generally common for them to stop breeding and miscarry at the 5th and 6th months. Even if the embryo can survive to natural production, future babies are highly likely to have health problems. Therefore, PGS is a highly valuable technique for pregnant women with advanced age and recurrent miscarriage.

PGD is genetic diagnosis mainly used to check whether embryos carry genes with genetic defects. After the sperm and egg combine in vitro to form a fertilized egg and develop into an embryo, genetic testing should be conducted before implantation into the uterus to prevent some genetic diseases in IVF. At present, pre implantation genetic diagnosis in China can diagnose some single gene genetic diseases, such as hereditary hearing loss, polycystic kidney disease, and so on. If parents suffer from this single genetic disease, they are highly likely to pass it on to the next generation. This type of examination is performed in the same way as PGS, but the laboratory does not examine chromosomes, but specific pathogenic mutant genes. By using PGD technology, it is possible to determine which embryos are normal, thus avoiding the inheritance of single gene diseases and giving birth to healthy babies.

What diseases can PGD/PGS screen for?

PGS mainly targets chromosomal issues. Each person has 23 pairs of chromosomes, of which 22 are autosomes, which are the same for both men and women, and 1 pair is a sex chromosome, which is different for men and women. For PGS screening, some hospitals generally only screen 5 pairs of chromosomes, which can be screened, including 13 (First Third Syndrome), 18 (Edward Syndrome), 21 (Down Syndrome), X (Duchenne muscular dystrophy), and Y (acute myeloid leukemia). However, in the United States, Each pair of chromosomes has corresponding diseases and can be fully screened.

PGD only targets genes with genetic defects, so there are many diseases examined by PGD, and there are many sub diseases under each major disease, such as Mediterranean anemia, hereditary ovarian cancer, breast cancer, congenital ichthyosis, etc.