Genetic Screening

Preimplantation Genetic Testing (PGT)

What is PGT?

Preimplantation genetic testing (PGT), either for the purpose of diagnosing a specific single gene or Monogenic condition (PGT-M, formerly known as PGD) or for the purpose of screening for abnormal chromosomal rearrangements known as Aneuploidy (PGT-A, formerly known as PGS) are techniques that allow embryos to be tested for such conditions prior to being transferred to a woman’s uterus for implantation.

Due to the techniques involved, PGT can only be 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 transferred to the uterus.

Who should consider genetic testing?

Without PGT-M and PGT-A, it is impossible for any embryologist to determine whether a given embryo is genetically or chromosomally normal respectively, because normal and abnormal embryos are usually indistinguishable from one another visually. In other words, it is possible for embryos that have chromosomal or genetic abnormalities to look very healthy. This is the reason why many couples will turn to PGT.

PGT-M: Couples who are both carriers of a known genetic disorder should strongly consider using PGD technology. Examples include when both couples carry sickle cell disease or cystic fibrosis, or if they carry certain genes that may increase the risk of their child developing cancer. Additionally, men with severe non-obstructive azoospermia or other forms of severe male infertility may consider PGT, if they are found to carry potentially harmful genes that can be passed to their offspring.

PGT-A: Couples who have experienced multiple pregnancy losses or IVF failures may want to consider utilizing PGT-A to screen their embryos for chromosomal imbalances or abnormalities. Such abnormalities are the most common reason that embryos fail to implant in the uterus. When such embryos do implant despite the abnormalities, the result is most often a miscarriage although, on occasion, the result is the birth of a child with developmental problems and/or mental retardation.

PGT allows embryos that are affected by genetic disorders or chromosomal abnormalities to be identified before they are transferred to the uterus. In other words, only genetically normal embryos with a complete complement of chromosomes will be considered for transfer, giving the best chance for an ongoing pregnancy and delivery of a healthy child.

What are the benefits from PGT?

  • 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 PGT-M 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 PGT-A to select chromosomally normal embryos, the chance of a pregnancy being healthy and going to term increases.

What procedures are involved?

Preliminary screening: Several screening tests are required for all IVF patients undergoing PGT, 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 281 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 PGT testing with their IVF cycle.

For patients who are pursuing PGT to test for a specific genetic condition, another DNA test is required to develop special genetic probes for the PGT-M 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 PGT 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: PGT 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 or more cells from each embryo for analysis.

Cell analysis: The biopsied cells are analyzed for chromosomal or genetic abnormalities. The 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 analysis allows the physician to identify the embryos that are free from the chromosomal or genetic abnormalities under study.

Embryo transfer: With the PGT results, embryos that are not affected with chromosomal or genetic conditions are transferred 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.

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