NGS (Chromosomal Screening)

Genetic and chromosomal examination of embryos before they are placed in the uterus is called PGD. Biopsy for examination can be performed at 3 different stages of embryonic development. A healthy embryo can be transferred to the mother after genetic analysis of the polar body from the oocyte, blastomer from the cleavage period embryo, or trophectoderm cells from blastocysts that have reached the fifth day as preferred.

Especially in advanced maternal age (38 years and over), pregnancy does not develop due to chromosomal abnormalities or pregnancy may end with miscarriage. In addition, prenatal diagnosis methods can be used to determine whether pregnancy is healthy in couples at high risk of transmitting a genetic disease to their children. Unfortunately, many couples have to bear the psychological, physical and financial burden of termination of pregnancy numerous times until they have healthy children. In terms of cost calculations, having a healthy baby with PGD is more advantageous than having a sick child when screening for both aneuploidy (chromosomal disorder) and single gene diseases.

Next Generation Sequencing (NGS) 

 The microarray technology used for PGT, or array CGH, is able to examine all chromosomes in the embryos by binding to approximately 4000 genetic markers scattered across the entire genome. However, it can detect it indirectly by comparing it to two known DNAs that are normal. In the next generation sequencing (NGS) technique, DNA amplification (DNA amplification) in each sample is enzymatically decomposed into millions of parts. After several steps, the sequences of these DNA fragments are read and the total number of readings per embryo is determined. NGS method is more advantageous because it provides a more sensitive and accurate examination than aCGH method and it can detect mosaism in embryos as low as 20%.

Embryos containing two or more cell populations with different chromosomal structure are called mosaic embryos. It is known that mosaic embryos are associated with decreased implantation and pregnancy rates, increased genetic anomalies and adverse pregnancy outcomes. The ability of aCGH technology to detect mosaicity at a rate of only 50% is considered a disadvantage compared to NGS technology, and it is possible to provide more accurate and realistic information to couples about their chances of pregnancy with the reported mosaicity rate sensitive to NGS.

Comprehensive Chromosomal Screening (CCS)

CCS allows the most healthy embryo to be found. At the same time, you don't need to transfer a large number of embryos to increase your chances of pregnancy, thus eliminating the risk of multiple pregnancies. It also increases the chance of successful implantation and prevents unnecessary embryo transfer. By understanding the embryo that cannot hold onto the mother's uterus, the transfer of that embryo is prevented, unnecessary treatments are prevented and the expectation of the expectant mother is not allowed to cause psychological problems. On the other hand, it significantly reduces the risk of miscarriage.

Array-Comparative Genomic Hybridization (a-CGH) 

Comparative genomic hybridization (CGH), which is an alternative to FISH method in which a limited number of chromosomes are examined, has made it possible to examine all chromosomes in embryos. However, a significant disadvantage of the technique is that the process is long. For this reason, after biopsy, embryos are frozen by vitrification and stored. Since vitrification is a successful freeze-thaw method and with the increasing experience of the laboratory, all possible procedures can be performed successfully especially on the 5th and 6th day blastocysts.

What is FISH (Fluorescence In Situ Hybridisation)?

 In the past, the FISH technique has been used extensively for the determination of aneuploidy (numerical chromosomal disorders) and translocations (structural chromosomal disorders).  As a result of the biopsy, fluorescence probes are attached to chromosomes of embryo cells and the signals obtained from these cells are examined by microscope and information is obtained about chromosome numbers. Panels consisting of 8,13,14,15,16,17,18,19,20,21,22, X and Y chromosomes and based on the detection of the most common chromosomal anomalies in spontaneous abortions are used, with this panel, the majority of chromosomal anomalies that may occur in embryos and cause pregnancy miscarriages could be detected. The disadvantage is the limited number of chromosomes that can be examined by this method. In addition, the possible misdiagnosis of chromosome signals in the FISH method is another disadvantage of this method. Therefore, this method is currently only used in translocation carrier pairs, if next generation sequencing, NGS or array method is not suitable.