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Genetic Screening of Embryo

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Preimplantation Genetic Testing for Aneuploidy (PGT-A)

PGT-A or Preimplantation Genetic Testing for Aneuploidies is the process of screening IVF embryos for chromosome abnormalities prior to transfer. Our goal is to increase the likelihood of achieving a successful pregnancy. This allows us to test your embryos before one is transferred to your uterus to ensure your baby won’t have any chromosomal irregularities. Embryos are either euploid (normal) or aneuploid (abnormal), and we will only transfer normal embryos. PGT-A has come a long way in the past couple of decades and is one of the most incredible advances in assisted reproductive technology.

If you plan on testing your embryos, we will have to freeze those embryos prior to transfer while we wait for results from the genetic lab. Mate partners with Igenomix, the best genetic lab in the country, which also offers free counseling for all Mate patients. Speak to your local clinic to set up this session. This process is called embryo banking. After your results come back, we will thaw a normal embryo and transfer that one in a procedure called FET (Frozen Embryo Transfer).

The carrier screening test during your fertility assessment will also let us know if you and your partner are both carriers for certain hereditary genetic diseases. If this test comes back that both you and your partner are carriers for a disease, your baby has a high chance of being affected. If both parents are carriers, their chance of having an affected child is 25%. If one parent has the disease, their chance of having an affected child is 50%. This will require further testing and evaluation to ensure only embryos without the disease are transferred. During PGT-A, all your embryos will be biopsied and screened for any chromosomal abnormality.

PGT-A is a preventative measure used to identify chromosomal abnormalities in the embryo, even if there’s no known evidence of an abnormality in either parent. The procedure screens for chromosomal abnormalities such as deletions or translocations; defects caused when an individual’s chromosomes are rearranged.

Each cell should contain 23 pairs of chromosomes, or 46 chromosomes in total. PGT-A testing assesses all 23 pairs of chromosomes, including the two sex chromosomes (X and Y) that determine the embryo’s sex. PGT-A testing can determine if there are any extra or missing copies of chromosomes in each embryo. Chromosome abnormalities are responsible for 60% of miscarriages.

Your age plays a critical role in the health of your eggs, and the health of your embryos is largely determined by chromosome abnormalities. Abnormalities are unlikely to implant and usually result in miscarriage or disorders. By the time you reach 35, 50% of your eggs have normal chromosomes, and by 40, only 10-15% of your eggs have normal chromosomes. We highly recommend PGT-A screening for any women above 35.

PGT-A has been shown to increase success rates of pregnancy, lower miscarriage rates, and avoid abnormalities. After your embryos are tested, the doctor will perform an FET or Frozen Embryo Transfer. This will usually be a few weeks after your egg retrieval, and we will need to prepare your body with hormones the week before to prepare your uterus for an embryo. We want the conditions to be just right for that embryo to stick.

What is preimplantation genetic testing (PGT)

Preimplantation genetic testing (PGT) refers to testing done on embryos “preimplantation” or before embryos attach to the uterine wall. This testing can be done as part of the IVF process because embryos are made outside of the body and can be biopsied. This testing is done on embryos after they have reached the blastocyst phase. This is because a blastocyst has been divided into the cells that will become the baby (the inner cell mass) and the cells that will become the placenta. The cells that become the placenta can be sampled and sent for genetic testing. 

 

What type of PGT testing can be done on embryos?

PGT-A stands for “preimplantation genetic testing for aneuploidy.”  Aneuploidy means “the wrong number of human chromosomes.” PGT-A looks at the number of chromosomes in an embryo which in humans should be 46. Having the wrong number of chromosomes (aneuploidy) or a large missing piece (deletion) or extra piece (duplication) most often results in the embryo not implanting or the early pregnancy miscarrying. An aneuploid embryo that does implant and grow can result in a child with a genetic disease such as Down Syndrome or Turner Syndrome. PGT-A is best used to improve the chance of a successful embryo transfer and reduce the risk of failed embryo transfers and aneuploid-miscarriages. PGT-A is most impactful as a screen for embryos when 1) the eggs came from a patient who is older than 35 and 2) there are many embryos to screen. 

PGT-M stands for “preimplantation genetic testing for monogenic” diseases. Monogenic diseases come from a single known gene mutation. Patients with known monogenetic disease may choose to undergo PGT-M to decrease risk to their offspring. Patients with known increased risk on expanded carrier screening may choose to undergo PGT-M to decrease risk to their offspring. 

PGT-SR stands for “preimplantation genetic testing for structural rearrangements.” PGT-SR is very specific to patients who have undergone specific genetic testing (karyotype) based on their medical history. A karyotype is often performed for patients with recurrent miscarriages. Sometimes a patient may have the right amount of chromosomes but the chromosomes are structured in an atypical way that leads to a higher risk of chromosomally abnormal embryos. If a patient is found to harbor an abnormally arranged chromosome, PGT–SR may be done to precisely test these embryos

Step 1: IVF + Embryo Banking

The first step required for PGT screening will be an IVF cycle. IVF is about a month-long process where your ovaries are stimulated to produce eggs. Those eggs are retrieved and then fertilized in our lab with sperm. Preimplantation genetic testing can be done as part of any IVF cycle whether patient, partner or donor eggs or sperm are used. 

 

Step 2: Tissue Biopsy

After about 5-7 days in the lab, fertilized eggs that have advanced to the blastocyst stage, now called embryos, are safe to be biopsied and that biopsy will be sent to Igenomix, our genetic testing partner. When an embryo reaches the blastocyst stage, the tissue biopsy will be taken. A tiny cell sample is taken from every embryo. A blastocyst has about 70-100 cells and has separated into the cells that will become the baby and the cells that will become the placenta (the trophectoderm). About 5 cells from this future placenta are sampled. All embryos will be frozen directly after the biopsy.

Step 3: Tissue shipping and embryo freezing

The frozen tissue sample will be placed in a tube and shipped immediately to Igenomix via FedEx. Your tissue samples are extremely well protected and ship under the highest priority. The embryos are frozen and kept at Mate while we await the PGT results from Igenomix. Your team will discuss the results of your report with you and Igenomix genetic counselors are also available for questions. 

Step 4: PGT at Igenomix lab

When your samples reach the Igenomix lab a comprehensive analysis of all 46 chromosomes will be completed using next generation sequencing (NGS). They will screen for any and all chromosomal abnormalities.

Step 5: Report

After your embryos are tested, the lab will send a report to Mate describing the status of each embryo in detail. Your provider will call to discuss the results of the report and next steps.

Step 6: Choosing an embryo and frozen transfer

Based on the report in-conjunction with input from yourself and your provider, the embryologist will choose the healthiest embryo to transfer. That embryo will be transferred into your uterus. Patients receive medications to optimize the uterine lining and receptivity for the embryo.