Recent studies show that PGD may improve a woman’s chance of a baby being born with an atypical number of chromosomes. PGD is an advanced genetics test used with in vitro fertilization to determine the status of an embryo’s chromosomes. This website will try to help educate you about PGD, if you need more information, or want to know whether this test is right for you, we encourage you to consult with your IVF specialist.
What you should know about chromosomes
A normal cell has 46 chromosomes in 23 pairs, half from each parent, which contain DNA- our genetic roadmap. The condition of having an embryo or zygote with either more or less than 46 chromosomes is called aneuploidy. Very often, the likelihood of aneuploidy increases with the age of the woman, but it can also occur in women under 35. Aneuploid embryos may have extra (called trisomy) or missing (monosomy) chromosomes. A baby carrying an extra or missing chromosome may be born with mental and/ or physical defects. Down syndrome is a common example.
How does aneuploidy affect my ability to conceive or maintain a pregnancy?
A chromosomal abnormality can prevent the embryo from attaching to the wall of the uterus, eliminating any chance of pregnancy. It may also cause the implanted embryo to stop developing, resulting in a pregnancy loss.
More than 50% of embryos from women who are 35 to 39 show chromosomal abnormalities, while those over 40 have a frequency of aneuploidy of 80% or higher. That’s why the percentage of older women becoming pregnant is so low.
Additionally, experts consider aneuploidy to be the biggest factor for Reccurent Pregnancy Loss (of miscarriages) among women 35 and older, responsible at least half the time.
How does PGD work?
The PGD procedure involves the biopsy of very early embryos, when they reach the stage of 6-10 cells something that occurs on the morning of the third day after fertilization. The IVF embryologist carries out the process by removing one or two cells from each embryo by micromanipulation. Each cell is usually representative of all cells from that particular embryo. The genetic material or DNA of these cells is analyzed allowing the detection of the genetic defect causing the inherited disease. Genetic mutations are detected by a process called the polymerase chain reaction the PCR whereas chromosomal abnormalities are spotted by FISH (fluorescent in situ hybridization). Following the analysis only the embryos that test normal are selected for transfer to the uterus on day 4 or 5 after oocyte retrieval.
In some cases polar bodies, two small cells produced by the ripening egg before fertilization, may be tested. This only provides genetic information from the egg. It will not detect abnormalities that may occur the egg is fertilized by sperm.
Which patients benefit from PGD?
Pre implantation genetic diagnosis is a relatively new technique that detects genetic and chromosomal abnormalities. PGD is mainly applied in couples at risk of transmitting an inherited disorder to their children, therefore avoiding the birth of a diseased child. In addition PGD is also useful in other patient groups who have lower chance of a successful outcome following IVF treatment. Virtually all couples over 35 with a history of repeated IVF failures are excellent candidates. Studies show that women who have Recurrent Pregnancy Loss, previous aneuploid conceptions, known chromosome abnormalities and single gene defects, can also benefit. In fact, studies have shown that PGD can double the chance of implantation of the embryo, reduce pregnancy loss as much as three-fold, and increase the likelihood of live births. Data even found a reduction in miscarriage among IVF patients who did not have recurrent miscarriage, from 23% to9%. In women with an average age of 40, they found that the chance of an embryo to successfully implant doubled.
Which disorders can be detected with PGD
At present PGD is offered for X-linked diseases, which only affect boys, like hemophilia A and muscular dystrophy, for numerical chromosome imbalances or aneuploides and finally for single gene disorders. The term single gene disorder refers to any one of the hundreds of inherited diseases caused by mutation (change) in a single gene. Common examples include cystic fibrosis, alpha and beta-thalassemia, myotonic dystrophy, sickle cell anemia, Duchenne muscular dystrophy and fragile x syndrome.
Is PGD 100% accurate and safe?
PGD, the only test available to determine aneuploidy, has an accuracy rate of over 90%, without apparently affecting the normal development of the embryo or fetus. It is estimated that the removal of one cell reduces the ability of the embryo to implant by less than 3%.
Despite its advantages, PGD has certain risks and limitations. The potential damage of the embryo during biopsy and the high frequency of chromosomal mosaicism, which is well documented in human embryos, could lead to a relatively small number of embryos left. So, couples seeking PGD should be aware of this phenomenon keeping in mind that the possibility of misdiagnosis always exists. Therefore, prenatal testing is still recommended via chorionic villous samples (CVS) or amniocentesis.
What is the cost for PGD?
There is an additional cost for PGD, above that of your IVF cycle. In addition to the removal of the cells from the embryo, a procedure performed at the IVF center, actual testing is done at a specialized laboratory.
Key features of the gene PGD service
- PGD can help parents avoid having to contemplate pregnancy termination
- Available for virtually all single gene disorders
- Diagnostic protocols tailored to individual patients
- Extremely high assay accuracy rates
- Rapid results
- HLA – typing available
Dr Paraschos participated in the world's first PGD.