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Pre-Implantation Genetic Diagnosis 

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. PGD is always used 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.

What patients benefit from PGD?
PGD may benefit 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 be offered PGD. PGD may also benefit women who are over 35 years of age. Finally, PGD may be offered to men with severe non-obstructive azoospermia or other forms of severe male infertility.

What are the benefits of PGD?

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 retardation. PGD allows embryos affected with chromosomal or genetic conditions to be identified before they are considered for transfer into the uterus. In other words, the embryos which are known to be unaffected with a chromosomal or genetic condition can be selected for transfer into the uterus, giving the best chance for a healthy pregnancy to be achieved

Without PGD, it is impossible for the embryologist to determine whether a given embryo is chromosomally or genetically normal. Normal and abnormal embryos are usually indistinguishable from one another visually. It is possible for embryos that have chromosomal or genetic abnormalities to look very healthy.

In summary, PGD can:
• 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 and genetic conditions?
Chromosomal conditions are caused by having either extra or missing genetic material or a rearrangement in chromosome structure. Some chromosome problems are severe and may result in miscarriage or failure to conceive. If a baby with a chromosome problem is born, he or she could have developmental problems and/or mental retardation. Women who are over 35 years old are at increased risk to have pregnancies affected with chromosomal conditions.

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. Couples who are known or suspected to be at risk for having a pregnancy affected with a genetic condition are candidates for PGD.

Why is maternal age a factor?
As a woman's age increases, her chance of having a pregnancy with a chromosome proble 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 going to term should increase.

What procedures are involved in PGD?
Preliminary screening: Several screening tests are required, including infectious disease screening, chromosome analysis, routine genetic screening based on ethnicity, and a semen analysis for the partner. For patients who are known carriers of certain genetic or chromosomal conditions, another blood test is required to develop special "genetic probes." These probes are designed to bind specifically to the genes or chromosomes under study.

IVF: This step involves the same procedures as a regular IVF cycle. The ovaries are stimulated to produce multiple eggs. When the eggs are mature, they are retrieved. These eggs are fertilized in the laboratory using a technique called intracytoplasmic sperm injection (ICSI), where a single sperm is directly injected into each egg. The resulting embryos are monitored as they develop.

Embryo biopsy: To analyze the embryos, they are biopsied on their third day of development, when they are composed of 6 to 10 cells. One cell is removed from each embryo for PGD analysis.

PGD analysis: The biopsied cells are analyzed for chromosomal or genetic abnormalities. The PGD analysis allows normal cells to be distinguished from cells affected with chromosomal or genetic conditions. The cells genetic material should match the genetic material of the other cells from the original embryo.

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