In the laboratory, doctors extract and examine a single cell, from a three-day-old embryo to determine whether the baby is at risk for genetic diseases and whether the baby will be male or female. Because some genetic diseases only affect one sex, gender can be an important factor in embryo selection.
PGD is a good alternative to more-traditional prenatal diagnoses because testing occurs before pregnancy. In contrast, traditional tests such as amniocentesis are usually performed between the 16th and 20th week of pregnancy. Amniocentesis is the extraction and testing of a small amount of amniotic fluid from the uterus. Doctors use prenatal diagnosis to detect a genetic disease or condition in a developing fetus. The advantage of PGD is that genetic testing occurs before pregnancy, not after, decreasing the likelihood genetic abnormalities.
The window of opportunity to biopsy an embryo is narrow. By day three, the embryo will be composed of between four and 12 cells that are still distinct from each other. Eventually the embryo begins to compact and individual cells lose their clear outline making them more difficult to biopsy.
Pre-implantation genetic diagnosis checks for hundreds of diseases.
The most frequently diagnosed diseases and recessive disorders detected by PGD are:
- Cystic fibrosis- Etathalassemia
- Sickle cell disease
- Spinal muscular atrophy
- Myotonic dystrophy
- Huntington's disease
- Charcot-Marie tooth disease
- Fragile X syndrome
- Hemophilia A
- Duchenne muscular dystrophy
Inherited genetic diseases can result in pregnancy loss, the birth of a child with physical or mental problems, or difficulty with embryo implantation after transfer to the mother’s uterus.
Women may benefit from PGD testing if they are experiencing any of the following:
- Three or more consecutive, spontaneous pregnancy losses
- Two or more unsuccessful IVF cycles
- Maternal age of 35 or older
- Inherited genetic disorders such as muscular dystrophy, Down syndrome, and hemophilia
A partner with male-factor in fertility such as a low sperm count or poor sperm motility may also benefit from PGD testing.
Two tests can help embryologists and geneticists decide which embryos to transfer:
1) Fluorescence in-situ hybridization (FISH) is a technique in which laboratory physicians count the number of chromosomes present in an embryo in order to eliminate those with possible abnormalities. FISH is limited to staining only eight to 12 of the 23 chromosome pairs in the cell, so not all abnormal embryos can be identified. Those chromosomes most likely to be found abnormal or result in live-born abnormal children are tested.
2) Polymerase chain reaction (PCR) refers to the technique of splitting chromosomes into pieces and then looking at the chromosomal fragments for abnormalities. PCR helps to identify specific gene disorders.
If a patient’s medical history indicates the possibility that embryos could be affected by genetic disease, your physician may recommend tested using PGD.
The Northwest Center for Reproductive Sciences partners with Dr. Santiago Munne, a pioneers in PGD technology. Dr. Munne performs the analysis of the embryonic cells at Reprogenetics, Inc. (www.reprogenetics.com)
