PGD

First performed in 1988, Preimplantation Genetic Diagnosis (PGD) is a procedure which is used to screen embryos during the In Vitro Fertilization (IVF) process a host of genetic diseases and disorders. It also allows infertile couples who opt for the procedure, to select the healthiest embryos for transfer, increasing the chances of having a healthy baby. This procedure was , and it has significantly improved since that time.

How is PGD Done?

Preimplantation genetic diagnosis PGD also referred to as Preimplantation Genetic Screening (PGS) or just “embryo screening.”, is used during an IVF cycle to identify specific embryos that carry abnormal genes. PGD can also be done to help select embryos of a particular sex, usually to avoid sex-specific diseases but it can also be done for family balancing. PGD is normally recommended for couples where one or both of the parents are a known carrier of a genetic disease (i.e. hemophilia or cystic fibrosis).

To perform PGD, a sample of three-day old embryos is carefully biopsied and then subjected to genetic screening. When the fertilized egg has reached about 6 to 10 cell stage, one or two cells (known as blastomeres) are gently removed from each embryo using a microsurgery technique similar to that used in ICSI. The DNA from the removed blastomeres is then analyzed for any genetic diseases or disorders. After the screening, a doctor can identify the healthiest embryos, and recommend one or more for transfer back  to the uterus of the  mother.

What Genetic Diseases Can PGD Test For?

Numerous diseases and disorders classified as either chromosomal disorders, single gene defects or sex-linked disorders can be tested for through PGD. Specific chromosomes are tested for specific disorders, including:

•  Chromosome 13: Breast and ovarian cancers, deafness, Wilson Disease
•  Chromosome 15: Marfan Syndrome, Tay-Sach’s Disease
•  Chromosome 16: Polycystic kidney disease, Alpha thalassemia
•  Chromosome 17: Charcot-Marie-Tooth Disease
•  Chromosome 18: Niemann-Pick Disease, pancreatic cancer
•  Chromosome 21: Down’s Syndrome
•  Chromosome X: Duchenne muscular dystrophy, Turner’s Syndrome, Fragile X Syndrome
•  Chromosome Y: Acute myeloidleukemia

A newer technology, called MicroArray, will soon allow for the assessment of all 23 chromosome pairs instead of the 9 or 10 chromosomes currently tested.

Why consider genetic testing with PGD ?

Unfortunately, for women ages 40-44, 50 -100% of their eggs may be genetically abnormal, causing infertility but also significantly increasing the risks for miscarriages and genetic birth defects. If patirnts are going to use their eggs (instead of donor egg), IVF is your best option so both egg quality as well as embryo quality can be evaluated and then a few of the healthiest looking embryos can be transferred to your uterus.

There are two main reasons to perform PGD. The first is in the case of a couple which is prone to genetic diseases. For example, if a mother knows that she carries hemophilia, she may opt for PGD and choose not to implant embryos which test positive for the disease. A wide variety of genetic diseases can be screened for with the use of PGD, including Huntington’s Disease, Fragile X Syndrome, and cystic fibrosis, among many others.

PGD is also performed to test for aneuploidy, an unusual number of chromosomes which could cause medical problems; the risk of aneuploidy is increased with older mothers. In the case of trisomy, someone has three chromosomes where there should be a pair, and in monosomy, a single chromosome is present, rather than a pair. Aneuploidy can cause a range of birth defects, and testing for it can allow people to eliminate embryos which may carry such birth defects.

PGD has some very definite benefits. For example, some embryos will fail to develop, due to profound genetic defects, and eliminating those embryos increases the chances of a healthy pregnancy. Parents can also use knowledge gained during PGD to inform health decisions for their children, and using PGD can reduce the need for additional IVF cycles by increasing the chance of producing a baby.

PGD can also benefit families where one child is sick and requires a stem cell transplant. Through PGD, a couple would be able to identify an embryo that would be a perfect match for their ailing child. Once the second child was born, stem cells from her umbilical cord would be collected at no harm to the baby and used for the transplant at no inconvenience to the child or mother, unlike bone marrow stem cells where collection is a painful and invasive procedure.

Who is Eligible for PGD?
 
The most common situations for recommending PGD include:

• Women age 35 or older  who have had miscarriages or failed IVF (2 or more despite normal quality embryos)
• Miscarriages (2 or more genetic or unexplained)
• Severe male factor (especially when testicular biopsy is needed to obtain sperm)
• Previous birth of a child with a single gene disorder (examples such as Cystic Fibrosis, Tay Sachs, Muscular Dystrophy, Hemophilia to name a few)
   

PGD is not a routine part of IVF. To have PGD done, couples will have to pay an extra fee, on top of their IVF fees. However, if you or your partner are a carrier of a genetic disease or disorder that PGD can test for, this fee may be worthwhile for the peace of mind PGD can offer you.

This brochure summarizes the highlights about PGD.