Preimplantation genetic diagnosis (PGD) is the diagnosis of a genetic condition prior to achievement of a pregnancy. Developed in the early 1990′s, preimplantation genetic diagnosis (PGD) is a way for couples to prevent a pregnancy affected by a genetic condition or chromosomal disorder. There are various types of PGD available, depending on the needs of the individual couple. This form of genetic testing is performed on eggs or embryos during an in vitro fertilization (IVF) cycle. The embryos that have been analyzed and are found to be normal are transferred into the woman’s uterus, where they will hopefully implant and result in the birth of a healthy child.
Currently, we are able to perform PGD for many genetic conditions, including single gene disorders and chromosomal abnormalities. At RGI, we have been performing PGD since it became available in 1990. We pioneered the polar body removal technology and are one of the most active centers offering PGD in the world. Our laboratory staff has extensive experience in the most current, state-of-the-art PGD techniques.
Most individuals have 46 chromosomes. Misdivision of egg or sperm cells can result in an embryo with too many or too few chromosomes. This incorrect number of chromosomes is called “aneuploidy.” Down syndrome, trisomy 18, and Turner syndrome are examples of common aneuploidies. These disorders do not typically run in families, but occur spontaneously and are very common in developing eggs and embryos. Up to 60% of early miscarriages are due to aneuploidy, and the risk for aneuploidy increases with a woman’s age. RGI offers testing for common chromosome abnormalities, as well as 24-chromosome microarray, the newest technology with the capability to test the entire chromosome complement of a cell. The purpose of PGD for aneuploidy is to increase a couple’s chance for pregnancy, reduce the risk for miscarriage, and improve the overall chance of bringing home a healthy baby after in vitro fertilization.
Some couples request PGD for a specific genetic condition that may run in their family, such as Tay-Sachs disease, cystic fibrosis, muscular dystrophy, sickle cell anemia, or Huntington’s disease. Our center has extensive experience in testing for many single gene disorders, including rare genetic syndromes. PGD can be performed for nearly all single gene disorders, as long as the specific genetic mutation is known. Single gene testing can be combined with aneuploidy screening, to maximize the chances of having a healthy baby.
We can also test for the HLA status of embryos. This is often requested by couples interested in having a child who is a healthy genetic match to a sibling in need of a bone marrow transplant. HLA testing can be performed while testing for a specific genetic condition, such as beta-thalassemia or Fanconi anemia.
For individuals who carry a translocation, inversion, or other chromosome rearrangement, PGD can be used to test eggs or embryos for the specific rearrangement. This greatly decreases the risk for miscarriage and/or the birth of a child with multiple congenital anomalies, which are associated with an unbalanced chromosome translocation, and increases the chance of a healthy and successful pregnancy.
Polar bodies are the by-products of egg division. These cells do not serve any role for the egg or embryo and will naturally degrade; however, they can be removed and tested to determine the genetic status of the egg. Polar body testing has the advantage of being more representative of the embryo’s chromosomes (compared to blastomere biopsy), but only tests for the maternal genetic contribution to the embryo. Polar body biopsy occurs at the Day 0 and/or Day 1 stage. Both polar bodies must be removed and tested in order to make an accurate diagnosis, and therefore, fertilization of the egg (to a certain degree) is necessary. For couples in which discarding fertilized embryos is not an option, cryopreservation (freezing) can occur prior to the fusion of the egg and sperm nuclei (2PN stage). Results of polar body testing are typically available in time for a Day 3 or Day 5 transfer in the same IVF cycle.
Blastomeres are cells from the Day 3 cleavage stage embryo, when approximately 6-8 cells are present. Each blastomere contains both maternal and paternal genetic information. Blastomere biopsy is the most common method of PGD testing, and involves the removal of one of these cells at the Day 3 stage. Since chromosomal mosaicism (where different cells within an embryo have different numbers of chromosomes) is highest at the Day 3 stage, blastomere testing may not always be representative of the genetic content of the embryo. Results of blastomere testing are typically available in time for a Day 5 transfer in the same IVF cycle.
A blastocyst is a Day 5/6 embryo that has developed to a specific stage. It contains two cell types – the inner cell mass, which eventually develops into fetal tissues, and the trophectoderm, which gives rise to the developing placenta and other “extra-embryonic” tissues. Blastocyst biopsy involves the removal of a small number of trophectoderm cells at the Day 5 and/or Day 6 stage for genetic testing. Testing the embryo at this stage has the advantage of reduced chromosomal mosaicism (where different cells within an embryo have different numbers of chromosomes) as well as improved DNA amplification, and therefore, improved test accuracy. In order to have sufficient time to obtain test results, blastocyst testing typically requires that embryos are cryopreserved (frozen) after biopsy, and healthy embryos are later thawed and transferred during a later cycle.
No, PGD does not replace prenatal testing, such as chorionic villus sampling (CVS) or amniocentesis, which are the standard of care for the diagnosis of a genetic disease or chromosome problem. PGD greatly reduces the risk of a pregnancy being affected with a genetic disease or chromosome problem, but it is not perfect. Prenatal testing is still recommended when a pregnancy is conceived. Our genetic counselors are available to discuss prenatal testing options with you.
Please contact our genetic counselors via email or call 773-472-4900 for updated pricing information. If you would like RGI to verify your insurance benefits, please contact our billing department at email@example.com.