Preimplantation Genetic Screening: Chromosomal Abnormalities

Preimplantation genetic screening (PGS) is the procedure used during preimplantation genetic diagnosis (PGD), mostly in combination with in vitro fertilization (IVF) and embryo screening. In vitro fertilization is the process of mixing and fertilizing the female eggs cells and sperm outside of the uterus, to be later implanted into the woman's womb. Oocytes (female cell involved in reproduction) are collected during the in vitro fertilization process to diagnose to see if there are any chromosomal abnormalities that could have been passed on in the genes from either of the parents or somewhere along the line of the family history. Women who have experienced several miscarriages or those that are aged over 35 to 37 years may also contemplate this type of screening.

Testing for Abnormal Chromosomes

PGS is used to test for abnormal chromosomes with in vitro fertilization three days after the fertilization (6 to 10 cell stage). If there are less embryos with chromosomal abnormalities, the in vitro fertilization procedure can achieve greater results. Two different techniques are used to test the embryo for genetic abnormalities. The PCR (polymerase chain reaction) technique for single-cell diagnosis allows for multiplication (DNA) for testing. With this technique, up to nine chromosomes can be tested. The embryos are then kept in a growth media to permit further division. Once the diagnosis is complete, the other embryos may be transferred again into the uterus for the in vitro fertilization treatment. With the FISH (fluorescent in situ hybridisation) technique, specially chosen fluorescent flexible devices (probes) are used to test the chromosomes. With this method, up to 10 chromosomes can be tested at one single time with glowing molecules (fluorochromes) that are able to attach to certain chromosomes.

Although preimplantation genetic screening does somewhat improve the rates of successful pregnancies, there is still the chance for error with misdiagnosed chromosomes. A new type of screening called comparative genomic hybridization (CGH) allows for a better study of the cells. The screening is done five days after, during the blastocyst stage. With later screening of the embryos, there is a higher likelihood of detecting chromosomal abnormalities, with medical studies indicating better success rates than preimplantation genetic screening.

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