Advanced Embryo Selection is a term used to describe a genetic test carried out on IVF embryos prior to transfer. AES does not diagnose any specific diseases but looks more generally at the genetic makeup of the embryo
In order to understand genetic testing of embryos we have to first grasp some basic cell biology; each cell in the body has 46 Chromosomes, except eggs and sperm which have 23 each. The egg and sperm combine to give the offspring a total of 46 chromosomes – half from the egg and half from the sperm. Each chromosome has many genes, or instructions, its length. The genes control characteristics like the hair or eye color of your baby.
Below is a diagram of normal male chromosomes, notice that the chromosomes are made up of pairs, two of each plus an XY for males and an XX for females (making a total of 46)
When an egg and sperm combine to form an embryo, there is sometimes an extra chromosome present; we see this with disorders such as Down’s syndrome where there are 3 copies of chromosome 21. The extra chromosome can come from either the egg or the sperm and this aberration can happen in natural conception. In addition to there being extra chromosomes, there may be chromosomes missing from the embryo. In most cases, either a loss or gain in chromosomes will result in an embryo that will not implant or grow normally.
If an embryo contains the wrong number of chromosomes, let’s say for example one is missing, then the cells in that embryo are missing a whole set of instructions. If there is an extra chromosome the cells will malfunction by following those instructions twice as often as they should.
It is possible to test each individual embryo created through IVF and count the number of chromosomes present. This is done by taking cells from the embryo on either day 3 or day 5 of development and carrying out tests on those cells to analyse their genetic makeup. The embryo can easily compensate for the removal of cells with a few cell divisions. In good hands, the biopsy of an embryo has little effect on its growth, although it may slow its growth down slightly.
Comparative Genomic Hybridisation (CGH):
CGH is a genetic test of eggs and embryos. It offers much more precise information than standard genetic testing has in the past. During CGH testing all chromosomes including X and Y are analysed and compared to a sample of “normal” DNA. The embryonic DNA is labelled green and the normal DNA is labelled red. If the embryo has a normal number of chromosomes, then there will be an equal amount of embryo (green) DNA and “normal” (red) DNA and the result should appear yellow.
If the embryo is missing a chromosome, that area appears more red (i.e. extra “normal” DNA). If the embryo has an extra chromosome that area appears greener. A computerized scanner interprets the results of the slides which are then verified by a trained scientist.
The advantage of doing CGH testing over traditional testing is that all 23 pairs of chromosomes can be tested and the results are more accurate when testing for aneuploidy. CGH may increase IVF pregnancy rates by helping to identify the Embryos most likely to produce a pregnancy, allowing them to be prioritized for transfer.
In most cases, chromosomal normal and abnormal embryos look identical down the microscope. Without genetic selection an embryologist cannot differentiate between them and it is possible that chromosomally abnormal embryos could be inadvertently transferred to the uterus. Most chromosomally abnormal embryos do not implant, or will miscarry shortly after implantation. By ensuring that embryos with a normal number of chromosomes are identified and given maximum chance of pregnancy
We here at KIC through our Infertility and Surrogacy Program try to provide you a Single Window Solution for all your queries in your journey to Parenthood.
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