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Prenatal Diagnosis

Preimplantation Genetic Diagnosis

Amniocentesis, CVS, and maternal serum screening are performed after a pregnancy is confirmed or in progress. In contrast, preimplantation genetic diagnosis (PGD) occurs before the embryo implants in the womb. This technique is performed on an embryo that has been derived from in vitro fertilization (IVF) and is growing in a laboratory dish. At about the 8-cell (day 3) stage, a cell is removed and the DNA and chromosomes are checked using FISH or a probe for a specific gene. If the cell is free of the defects being probed, the remaining 7-celled embryo is implanted into the woman, where it continues development.

A doctor at Baltimore University Hospital has inserted the needle that will extract amniotic fluid from this patient's placenta. The fluid obtained from the amniocentesis will be analyzed to detect genetic disorders.

The first PGD was done in 1989, when it was used to enable families with X-linked disorders to select a girl, who would not be affected by the condition. Then it was used to conceive Chloe O'Brien, a youngster free of the cystic fibrosis that affected her brother. PGD attracted widespread public attention in 2001, when a Minnesota couple, Lisa and Jack Nash, conceived their son Adam so that his umbilical cord stem cells could be used to cure his sister Molly's Fanconi anemia. Adam not only had not inherited Fanconi anemia, but he was also a tissue match for Molly, saving her life.

PGD has been used to eliminate embryos with a variety of single-gene disorders, including metabolic disorders, dwarfism, cystic fibrosis, hemophilia, muscular dystrophies, and several other genetically inheritable diseases. The technique is being increasingly used in couples for whom IVF has repeatedly failed because they manufacture eggs or sperm that have abnormal numbers of one or more chromosomes. PGD enables physicians to sort through embryos to identify and transfer those few that have normal chromosomes. PGD has about a 66 percent success rate for identification of genetic disorders.

Additional topics

Medicine EncyclopediaGenetics in Medicine - Part 3Prenatal Diagnosis - Viewing Chromosomes, Less Invasive Methods, Preimplantation Genetic Diagnosis, Genetic Counseling And The Ethics Of Prenatal Diagnosis