PGD-PGS Preimplantation Genetic Diagnosis Screening
Pre-implantation genetic diagnosis (PGD) and Pre-implantation genetic screening (PGS) is a way for intended parents to prevent a pregnancy affected by a genetic condition or chromosomal disorder. This advanced testing identifies specific embryos that are free of abnormalities before they can be transferred into a woman’s uterus (womb) by way of in-vitro fertilization (IVF).
In addition to safeguarding against genetic diseases or disorders, PGD-PGS can also assist intended parents interested in having a child who is a healthy bone marrow or blood match to another family member by selecting embryos that are an HLA (human leukocyte antigen) match. HLA are the proteins on white blood cells that are matched in order to do a blood or bone marrow transplant.
Limitations to PGD-PGS
The risk of a misdiagnosis resulting in a fetus or baby with chromosome abnormalities after PGD-PGS is less than 1 percent. However, PGD-PGS is unable to study every chromosome and does not guarantee the birth of a healthy baby.
Because of these limitations, prenatal testing later in pregnancy is strongly advised in order to confirm the diagnosis and review the number and structure of all the chromosomes.
Additional Prenatal Testing
Chorionic Villous Sampling (CVS) is a procedure done late in the first trimester (between 9 and 11 weeks) that takes cells from the placenta and analyzes them for chromosomal abnormalities.
Amniocentesis is a procedure usually done between 15 and 20 weeks of pregnancy that takes fluid from around the baby and analyzes the baby’s cells in the fluid for chromosomal abnormalities. Your obstetrician can provide you with more information on these important tests.
Also based on cell development, some PGD-PGS testing may yield no diagnosis, partial diagnosis or even sometimes result in no normal embryos for embryo transfer. The likelihood that this will happen is often dependent on your age.
Single gene defects are genetic disorders due to inheritance patterns (recessive or dominant) and can be passed on if they are prevalent in your family. Some of the more common single-gene disorders include Tay-Sachs disease, cystic fibrosis, muscular dystrophy, fragile X syndrome or spinal muscular atrophy.
Chromosome translocations are abnormalities of a chromosome’s structure. Translocations occur when fragments or pieces of a chromosome break off and rearrange onto a different chromosome or rearrange within itself. Translocations can either be balanced or unbalanced. Balanced translocations contain all genetic information, but pieces of chromosomes have switched places resulting in no gain or loss of chromosome material. Unbalanced translocations result in an individual having more or less chromosomal material, thus causing genetic defects.
Aneuploidy is a condition in which a cell has an incorrect number of chromosomes—too many or too few. Chromosomes contain all of your genes and DNA, the building blocks of the body. Humans have 46 chromosomes, or 23 pairs. These include chromosomes 1 to 22 (the autosomes) and chromosomes X and Y (the sex chromosomes.) During fertilization, an embryo receives 23 chromosomes from the sperm and 23 chromosomes from the egg to form either 46,XY (normal male) or 46,XX (normal female). Aneuploidy disorders do not typically run in families and can include Down syndrome, Trisomy 18, Trisomy 13 and Turner syndrome.