PGD, or preimplantation genetic diagnosis, is a test to detect genetic disorders. It is conducted during artificial insemination procedures, i.e. before pregnancy is established. PGD testing is usually conducted before the fertilized eggs are implanted in the woman's uterus, i.e. it is used for testing wherein in vitro fertilization is used for artificial insemination.
PGD tests are useful for detecting:
- Ovarian cancer
- Marfan syndrome
- Pancreatic cancer
- Breast cancer
- Muscular dystrophy
If the PGD test indicates towards the possibility of a genetic disorder, the marked embryo is eradicated.
Understanding Alternatives to Preimplantation Genetic Diagnosis Testing
Alternatives to PGD testing are discussed with the patient when it is felt that a more precise form of genetic testing is critical. This is usually found among women who have a history of genetic disorders or congenital defects in the family.
The two most common alternatives to PGD testing are:
- Chorionic Villus Sampling or CVS
- Amniocentesis
These two tests are commonly referred to as Prenatal Diagnostic Testing. Often, they are used as follow-up testing, i.e. they aren’t used as a replacement for PGD testing but as more elaborative, follow-up tests.
Chorionic Villus Sampling
Chorionic villus sampling, or CVS testing, is conducted during the early part of pregnancy. This screening can identify more than two hundred types of congenital diseases. A CVS is usually performed during 10-to-12 weeks after the last menstrual cycle. Here, a small sample is obtained from the placental tissue, called chorionic villus. CVS is a faster way of confirming genetic abnormalities than amniocentesis as the confirmatory reports are provided much earlier (within two weeks). Further, this test can be conducted much sooner than amniocentesis. The sample obtained from the patient is used to analyze the DNA structure, the chromosomal links and other factors like enzymes in the embryonic fluid. CVS can detect some typical genetic disorders like Tay-Sachs disease.
Chorionic Villus Sampling Limitations
This testing doesn’t provide very clear indications regarding any defects in the formation of the fetus’ body parts. Further, CVS is unable to detect complicated genetic disorders like neural tube abnormality or Rh Incompatibility. If any of these genetic defects are indicated during CVS, an amniocentesis is advised.
Amniocentesis
Amniocentesis is performed during pregnancy by testing the embryonic fluid that is present in the nutritive sac surrounding the developing embryo. This is regarded as the most dependable test for identifying possible birth defects or chromosomal disorders. This test can also confirm the proper development of various organs like the lungs and other problems that might induce the need for forcing an early delivery. The test is performed on an outpatient basis, wherein the doctor uses an ultrasound to locate the fetus. An anesthetic is used to numb the point of injection upon the protruding belly of the mother. The attending physician injects a fine needle and extracts a small sample of the embryonic fluid for testing.
Amniocentesis Applications
Amniocentesis is a reliable test for decoding the presence of:
- Spina bifida (spinal cord developmental defect)
- Down Syndrome (Trisomy 21)
- Anencephaly (Cephalic Disorder)
- Genetically-inherited metabolic disorders, like albinism
