Genetic Screening

Preimplantation Genetic Diagnosis (PGD)

What is PGD?

Preimplantation genetic diagnosis (PGD) is a technique that allows embryos to be tested for certain conditions prior to being transferred into the uterus of a woman. PGD can be used to diagnose a wide variety of chromosomal and genetic conditions in embryos. Due to the technique involved, PGD is possible only in conjunction with in vitro fertilization (IVF). IVF allows fertilization of the eggs to take place in a laboratory, where the resulting embryos can be tested before being placed into the uterus.

Who benefits from PGD?

PGD helps couples who are known carriers of chromosomal or genetic conditions, as there is a high likelihood that their embryos could be affected by these conditions. Couples who have experienced multiple pregnancy losses or multiple IVF failures may also benefit from PGD. Because PGD can be used to detect chromosomal conditions such as Down syndrome, women who are over 35 years of age are good candidates for PGD since they are at higher risk to have pregnancies affected by this type of condition. Finally, PGD may be offered to men with severe non-obstructive azoospermia or other forms of severe male infertility.

How can PGD help a couple?

Embryos with certain chromosomal or genetic conditions are known to have developmental problems. These embryos could fail to implant in the uterus, result in miscarriage, or result in the birth of a child with developmental problems and/or mental disabilities. PGD allows embryos affected with chromosomal or genetic conditions to be identified before they are considered for transfer into the uterus. In other words, PGD enables a woman, with her physician, to select which embryos to transfer into her uterus. In this way, the embryos that are known to be unaffected with a chromosomal or genetic condition can be chosen for transfer, giving the best chance for a healthy pregnancy to be achieved.

Without PGD, it is impossible for an embryologist or physician to determine whether a given embryo is chromosomally or genetically normal. Normal and abnormal embryos can be indistinguishable from one another visually. In fact, it is possible for embryos that have chromosomal or genetic abnormalities to look very healthy, particularly in the early stages of development.

In short, PGD allows a patient to choose her healthiest embryos for transfer into the uterus. The benefits of PGD are to:

  • increase implantation rates and pregnancy rates
  • reduce pregnancy losses
  • reduce the chance of having a child with a chromosomal or genetic condition

What are chromosomes and genes?

Our bodies are made up of cells, and within our cells are our chromosomes. Chromosomes are the structures in the cells that contain our genes, or our genetic material. Our genes are made of DNA and contain the information that allows our bodies to develop and function normally. We inherit our chromosomes and genes from our parents, via the sperm and egg.

What are chromosomal conditions?

Chromosomal conditions are caused by having extra or missing genetic material or a rearrangement in chromosome structure. Some chromosome problems are severe and may result in miscarriage or failed implantation. If a baby with a chromosome problem is born, he or she could have developmental problems and/or mental disabilities. Women who are over 35 years old are at increased risk to have pregnancies affected with chromosomal conditions.

What are genetic conditions?

Genetic conditions refer to specific diseases usually caused by a single gene. A person can have a genetic condition if one of his or her genes does not function properly. There are many different types of genetic conditions. For many of these conditions, a couple is at risk of having a baby with the condition only if both members of the couple are carriers of the condition. Couples who are determined to be carriers of the same condition are candidates for PGD because they have an increased risk to have a baby affected with the genetic condition.

Why is maternal age a factor?

As a woman’s age increases, her chance of having a pregnancy with a chromosomal condition also increases. Because women are born with all the eggs they will have in their lifetime, a woman’s eggs are as old as she is. It is thought that older eggs are more likely to contain “mistakes” in their chromosome number, i.e. extra or missing chromosomes. These extra or missing chromosomes can cause problems in the embryo and developing fetus. Affected embryos are less likely to attach to the uterus or go to term. Most result in miscarriage.

In women 35 and older, approximately 35% of pregnancies are miscarried. Chromosome problems are the cause in 50% or more of these losses. By utilizing PGD to select chromosomally normal embryos, the chance of a pregnancy being healthy and going to term increases.

What procedures are involved in PGD?

Preliminary screening: Several screening tests are required for all IVF patients undergoing PGD, including infectious disease screening, chromosome analysis, routine genetic screening based on ethnicity, and a semen analysis for the partner. Expanded genetic screening is also available to interested couples. Expanded genetic screening allows a couple to undergo carrier testing for up to 180 different genetic conditions. If the results of the expanded genetic screening reveals that both members of a couple are carriers for the same genetic condition, they may qualify for complementary PGD testing with their IVF cycle.

For patients who are pursuing PGD to test for a specific genetic condition, another DNA test is required to develop special genetic probes for the PGD analysis. These probes are specifically designed to bind to the individual couple’s DNA and are used to test their embryos for the genetic condition under study.

IVF: All patients who pursue PGD follow the same procedures as patients undergoing regular IVF cycles. The ovaries are stimulated to produce multiple eggs. When the eggs are mature, they are surgically retrieved. These eggs are fertilized in the laboratory and the resulting embryos are monitored as they develop.

Embryo biopsy: PGD involves testing the chromosomes and/or genes of embryos. To analyze the embryos, they are biopsied on their third or fifth day of development, depending on various criteria. The biopsy involves removing one cell from each embryo for PGD analysis.

PGD analysis: The biopsied cells are analyzed for chromosomal or genetic abnormalities. The PGD analysis allows normal biopsied cells to be distinguished from biopsied cells affected with chromosomal or genetic conditions. Because the biopsied cell’s genetic material should match the genetic material of the original embryos, the PGD analysis allows the physician to identify the embryos that are free from the chromosomal or genetic abnormalities under study.

Embryo transfer: With the PGD results, embryos that are not affected with chromosomal or genetic conditions aretransferred into the patient’s uterus. One to three of these embryos can be transferred with the hopes of achieving a healthy singleton pregnancy. Any extra healthy embryos can be cryopreserved for future use.