The basics of embryo biopsy and genetic testing
The first step to genetically test an embryo prior to transfer is the embryo biopsy, in which a few cells are removed from each embryo on Day Five. Waiting until Day Five, when the embryo has hundreds of cells, allows for the removal of more cells, increasing testing accuracy.
To retrieve the cells, the embryologist uses a laser to create an opening in the outer shell of the embryo, taking care not to damage the embryo as cells are removed. The sample cells are then sent to a separate genetics laboratory, where geneticists perform one of two different types of tests.
- Preimplantation genetic screening (PGS), or aneuploidy screening, determines the chromosomal composition of the embryo.
- Preimplantation genetic diagnosis (PGD) can diagnose an embryo with a specific genetic disease.
While the processes for these two different types of embryo biopsy tests are different, their goal is the same: Preventing the transfer of genetically abnormal embryos to the woman’s uterus.
PGS embryo biopsy
PGS is the most common type of testing associated with embryo biopsy. Each normal embryo contains 46 chromosomes, with 23 contributed by the sperm and 23 by the egg. In this test, geneticists are checking to be sure that the embryo has 46 normal chromosomes, which means it is a normal embryo. If there are fewer than or more than 46, the embryo is abnormal, or aneuploid.
PGS embryo biopsy can also reveal an embryo’s sex. Two types of chromosomes – X and Y – determine the gender of the embryo, while pairs of autosomes (1-22) determine almost everything else. A female embryo will have XX chromosomes, while a male will have XY. The sperm contributes one sex chromosome (X or Y) and 22 autosomes, while the egg contributes one sex chromosome (X only) and 22 autosomes.
PGD embryo biopsy
PGD embryo biopsy is typically ordered when one or both hopeful parents are known or suspected to carry a gene responsible for a specific disease. Geneticists can diagnose the disease in the embryos by looking for the presence of that specific gene, so that only disease-free embryos can be transferred. More than 1,000 diseases can be diagnosed through PGD, and that number is constantly growing as genetic research continues.
Unlike PGS, which examines comparatively large chromosomes, PGD focuses on single genes, composed of a small piece of DNA that is made from combinations of four basic molecules hooked together in a precise sequence.
To detect the presence of a gene, the DNA sequence that contains it must first be separated from the chromosome using enzymes. Then, that piece of DNA is copied millions of times through a process called polymerase chain reaction. Once the necessary amount of DNA has been produced, a probe specific to that gene is combined with the DNA. If the gene is present, it can be seen by a process that allows visualization of the probe attached to the gene.