What is Preimplantation Genetic Diagnosis (PGD)?
- Posted on: Jan 22 2013
Pre-implantation Genetic Diagnosis (PGD) refers to procedures that are performed on embryos or eggs prior to implantation or fertilization. PGD is considered another way to check for problems in a baby before it is born (prenatal diagnosis). When used to screen for a specific genetic disease, its main advantage is that it avoids having to decide if a couple wishes to undergo a pregnancy termination if the baby they are carrying is not free of genetic disease. PGD is used with assisted reproductive technology, and requires in vitro fertilization (IVF) to obtain oocytes or embryos for evaluation.
The term pre-implantation genetic screening (PGS) is used to denote procedures that do not look for a specific disease but use PGD techniques to identify embryos at risk for aneuploidy (see description below). So both PGD and PGS should be referred to as types of embryo screening.
The benefit of using PGD and PGS is testing occurs on the embryo ( or egg), and couples are aware of genetic defects in the embryo before implantation occurs, before they become pregnant. Couples can decide to not implant the abnormal fetal tissue, therefore preventing pregnancy with an abnormal child. This avoids the dilemma of having to decide what to do if the results of a pregnancy diagnosis test, such as an ultrasound or amniocentesis, that comes back abnormal.
PGD testing occurs commonly for:
∙ Alternative to amniocentesis and pregnancy termination for chromosomal abnormalities (aneuploidy testing)
∙ Female carriers of X-linked diseases
∙ To test embryos in those who are a carrier or have a genetic diseases (sickle cell, cystic fibrosis, Huntington chorea, spinal muscular atrophy etc) to make sure that the baby does not have the disease (single gene disorders)
∙ Carriers of structural or numerical chromosomal abnormalities (such as balanced translocations)
∙ Cancer disposition (carriers of BRAC genes)
∙ HLA matching
∙ Sex discernment for carriers of X linked disorders (Hemophilia, Rett syndrome) or family balancing
∙ IVF patients with a poor prognosis
– Advanced maternal aging (>38 years of age)
– Repeated implantation failures (Failed cycles ≥ 2)
– Recurrent pregnancy loss (≥ 3 consecutive spontaneous losses)
– Severe male factor (< 1million motile sperm) where risk for having a chromosomal abnormalities
Aneuploidy is the medical term that describes a chromosome defect. A healthy individual has 46 chromosomes that are paired together and are named 1-22 according to size. The two chromosomes left are the sex chromosomes X and Y. Males have an X and a Y chromosome while females have two X chromosomes.
PGD tests specific chromosomes that are known to cause defects such as Down syndrome, Turner syndrome, Trisomy 18 and 13. With the newest technology all chromosomes can be screened for abnormalities. When this testing is done about 50% of embryos in women under 35 are found to be abnormal and 60-70% of embryos in women over 35 are abnormal.
Conditions such as Down syndrome are caused by an extra chromosome 21. This abnormality can cause severe mental retardation and dysfunction of major organs. Down syndrome is sometimes referred to as Trisomy 21, because there are 3 of chromosome 21 instead of 2.
Turner syndrome is caused by a missing sex chromosome. Most pregnancies that are affected by Turner’s end in miscarriage. In the rare instance of a baby going full term it usually suffers from cardiac defects and future infertility.
Trisomy 18 and 13 are each caused by an extra copy of chromosomes 18 and 13. Most babies born with one of these disorders die within their first few months of life. Effects include mental retardation and other birth defects.
Women who are at least 35 years of age should consider chromosome PGD testing as the incidence of giving birth to a baby with a chromosome defect increases significantly with age.
Single Gene Disorders
There are literally hundreds of single gene disorders. These disorders are caused by a gene mutation that affects a person’s DNA. Common single gene disorders that are tested for include cystic fibrosis, sickle cell anemia, muscular dystrophy, and Tay-Sachs disease.
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