Preimplantation Genetic Diagonsis
Preimplantation genetic diagnosis (PGD) and preimplantation genetic screening (PGS) are ways to safeguard against genetic diseases or disorders. Intended parents can prevent a pregnancy affected by a genetic condition or chromosomal disorder. This advanced testing can identify specific embryos that are free of abnormalities before they are transferred into a woman’s uterus (womb) by in vitro fertilization (IVF).
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. Technology makes it possible to select embryos that match HLAs (human leukocyte antigen), the proteins in white blood cells that are matched in order to do a blood or bone marrow transplant.
PGD-PGS can test for and potentially prevent many diseases and genetic disorders, including:
Single Gene Defects
These genetic disorders are caused by inheritance patterns (recessive or dominant) and can be passed on if they are prevalent in families. Some of the more common single-gene disorders include Tay-Sachs disease, cystic fibrosis, muscular dystrophy, fragile X syndrome or spinal muscular atrophy.
These 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. 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.
This condition involves a cell with an incorrect number of chromosomes—too many or too few. Chromosomes contain all 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.
Limitations to PGD-PGS
The risk of a misdiagnosis resulting in a fetus or baby with chromosome abnormalities after PGD-PGS is less than two 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 villus sampling (CVS) is a procedure done late in the first trimester (between nine 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 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.