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Preimplantation Genetic Diagnosis (PGD)

Preimplantation genetic diagnosis (PGD) can be used to determine whether embryos produced by in vitro fertilization carry the gene for a specific genetic disease (such as cystic fibrosis, muscular dystrophy etc). This diagnosis can be made before embryos are transferred to the uterus for the establishment of a pregnancy. PGD greatly reduces the risk that a couple will pass the disease onto their child.  Thousands of clinical PGD cycles have been performed worldwide, resulting in the birth of hundreds of healthy babies.

In addition, PGD can also be used to determine if embryos, produced by In Vitro Fertilization (IVF), have chromosomal disorders.  The chromosomal disorders can prevent embryo implantation, lead to pregnancy loss or result in the birth of a child with physical problems and/or mental retardation. This technique is offered primarily to couples when one partner has a chromosome translocation or other chromosome abnormalities. It is also offered to women with recurrent miscarriages and women of advanced maternal age undergoing IVF. In all these situations PGD reduces the risk of achieving a pregnancy that will miscarry due to chromosome abnormalities or from having chromosomally abnormal offspring.

Why do couples choose PGD?

A. Couples who have a high risk of having a child with a severe genetic disease may choose PGD for many reasons, including:

• loss of a child from the disease
• fetus from a previous pregnancy was affected by the genetic disease
• objection to termination of pregnancy 

B. Couples who have a high risk of having a child affected by chromosomal disorder choose PGD for many reasons, including:

• pregnancy loss that was found to be due to a chromosomal disorder
• one partner carries a known chromosomal disorder
• multiple pregnancy losses after spontaneous conception 

Genetic counseling is an important step to ensure that PGD is the right option for you.

What does the process involve?

In Vitro Fertilization (IVF) is the first step prior to PGD.

• Medications are used to suppress a woman's natural menstrual cycle.
• Her ovaries are then stimulated with medications to produce multiple follicles, each of which may contain an oocyte (egg).
• The eggs are retrieved from the woman’s ovary by a needle placed in the vagina
• In the lab, the eggs are placed with the male partner's sperm, in a special culture medium that allows fertilization and the growth of high-quality embryos.

PGD

After three days in culture the embryos typically reach the eight-cell stage. One to two cells are removed from each embryo on day 3 and analyzed by one of the techniques described below:

A. Testing for the genetic mutation is performed only on cell(s) removed from each embryo.  The cells are tested by a very powerful technique called fluorescent polymerase chain reaction (F-PCR) that has the capacity to make millions of copies of the piece of DNA of interest so that a reliable diagnosis can be made on each embryo. The product from the F-PCR reaction is then tested for the presence or absence of the known parental mutation(s) using a range of genetic techniques. Currently this testing is performed at Genesis Genetics Institute.

B. For examining chromosomal defects the cells are analyzed using a technique call fluorescence in-situ hybridization or FISH.  This technique uses probes, small pieces of DNA that match the chromosomes of interest to study the cell’s chromosomal make-up.  Each probe is labeled with a different fluorescent dye.  These fluorescent probes are applied to the cell and attached to the chromosomes.  Under a fluorescent microscope, the geneticist counts the chromosomes, and, for translocation cases, can distinguish normal cells from cells with an unbalanced translocation. 

Embryo Transfe

• The results of the tests are reported by the morning of the day of embryo transfer (five days after oocyte retrieval and IVF).
• Of the embryo(s) not affected by the genetic disease, the best quality embryo(s) will be selected for transfer to the uterus.  If additional good quality, unaffected embryos are available, they may be cryopreserved for a future embryo transfer.

Does PGD replace prenatal testing?

No, PGD does not replace prenatal testing, such as chorionic villus sampling or amniocentesis. PGD provides diagnostic information based on the analysis of a single cell.  Therefore prenatal testing is still recommended, as this is currently the standard-of-care.

For more information, please contact:

Amy Sparks, PhD
University of Iowa Hospitals and Clinics
Department of Obstetrics and Gynecology
Reproductive Testing and In Vitro Fertilization Laboratory
565 Medical Research Facility
200 Hawkins Drive
Iowa City, IA 52242

phone: 319-384-8349
amy-sparks@uiowa.edu or anuja-dokras@uiowa.edu