The number of chromosomes being less or greater than normal is one of the most common chromosomal anomalies which is known as aneuploidy. Preimplantation Genetic Screening (PGS) is to evaluate chromosomes for aneuploidy before placing the embryos into the uterus. Aneuploidy can occur both in an egg and a sperm, and also in an embryo during fertilization stage. Aneuploidy can cause a problem with physical / mental development depending on the type of present chromosomal anomaly. PGS method is intended to identify embryos that do not present any anomalies and transfer in order to achieve a healthy baby.
Some of the embryos achieved with test tube baby cannot be implanted in the uterus while the development of others are prematurely ended, and some may not be able to continue developing until birth process even though they overcome these processes. As a result 20% of the pregnancies could result in miscarriage as in natural pregnancies. Chromosomal disorders have the largest role among causes of failures of embryo development. The most common chromosomal anomalies in miscarriage: trisomy (presence of three chromosomes instead of one) of chromosomes of 13, 16, 18, 21 and 22; monosomy (presence of a copy of chromosome); anomaly of the number of sex-determining chromosomes (X and Y).
Preimplantation Genetic Diagnosis (PGD) is applied on couples who are at risk for cystic fibrosis, thalassemia, or balanced structural translocation. Balanced structural translocation causes genetically unbalanced gamete (sperm or egg) formation and if these two join, genetic material will be either deficient or excessive, resulting in formation of an embryo with abnormal number of chromosomes. Such case in an embryo could result in death of the embryo, miscarriage, or delivery of a child with serious medical problems.
Following fertilization, the embryo is monitored for 3 days until it enters the phase with 8-10 cells in the medium. One or two cells are taken from blastomer of these 3-day embryos with biopsy and particular chromosomes are subjected to chromosomal analysis. PGS/ PGD process is applied on chromosomes (13, 16, 18, 21, 22, X and Y) where chromosomal anomaly commonly occurs in miscarriage and live birth.
Embryos undergone a biopsy are monitored until 5th or 6th day until blastocyst stage. Only the embryos with no anomalies will be transferred to the uterus.
When performing PGS/PGD process on chromosomes, Fluorescence In Situ Hybridization (FISH) method is used to attach particular probes (small pieces of DNA) to the specific areas to be analyzed. Each probe is labeled by a different fluorescent paint. These fluorescent probes are enabled to react with cells taken by biopsy to allow hybridizing with particular chromosomes. A geneticist uses a fluorescent microscope to evaluate color signals (one, two, or three) on particular chromosomes. One signal is considered to be monosomy, two signals mean diplosomia, and three signals mean trisomy. So, aneuploidy is determined on particular chromosomes.