Preimplantation Genetic Testing

In vitro fertilization (IVF) is a process where embryos are created externally to the body and grown in a laboratory to optimize success and minimize variables that prevent conception.  
 
Doing IVF to minimize the chance of having a child with a genetic disorder is the same process as doing IVF for infertility. The IVF process can be used for fertility purposes as well as minimizing the chance of having a child with a genetic disorder. The difference is that individuals or couples with infertility try less expensive means of getting pregnant first. Individuals or couples doing IVF for genetic reasons skip straight to IVF where embryos can be tested for a genetic issue.

Stims/Controlled Ovarian Hyperstimulation 

The IVF process starts with the process of retrieving eggs in controlled ovarian hyperstimulation (stims) for the female or egg-having partner. In a regular ovulatory cycle, a sequence of hormonal changes leads to the maturation and release of a single mature egg. In an IVF stims cycle, medication is given for about 10-14 days to create several mature eggs at once. When the follicles have reached optimal size, the egg retrieval will be scheduled.  

1a – Egg retrieval 

Egg retrieval comes after the hyperstimulation process. The egg retrieval is a minimally invasive surgery performed with light sedation. The Reproductive Endocrinologist will retrieve the eggs from the ovaries using a needle through the vaginal wall. This is an ultrasound-guided procedure and takes about 20 minutes to complete. The eggs are taken back to the IVF laboratory.  

1b – Sperm collection 

The male partner will provide a sperm sample within 2 hours of the egg retrieval procedure. This can be done at the fertility clinic, at home on the day of the retrieval, or a frozen semen sample can be used.  

2 – Fertilization 

The embryologist will fertilize each egg separately with sperm from the partner or sperm donor. In order to reduce the risk of artifact DNA present within the embryo (which may make genetic testing more difficult), the majority of labs in the United States require the use of intracytoplasmic sperm injection (ICSI). ICSI is a procedure where a singular, good-quality sperm is injected directly into a mature egg.  

3 – Embryo growth 

Cells should multiply day by day to 2 cells, 4 cells, 8 cells, and by 5 days after fertilization, the embryo should contain about 200 cells, known as a blastocyst. At this point, there should be cellular differentiation, where there is a clear inner cell mass (which becomes the fetus) and an outer part, trophectoderm (which becomes the placenta). It can take 5-7 days for an embryo to become a blastocyst. 

4 – Embryo biopsy 

Biopsies are most safely taken from embryos that have successfully reached the blastocyst stage. In order to do this, the embryologist will take a biopsy of about 5 cells from the outer part of the trophectoderm (the part of the embryo that becomes the placenta). The biopsy is sent to the genetics company for analysis. 

5 – Embryo cryopreservation 

The blastocyst stage embryo, which is about 200 cells, is frozen after it is biopsied. The embryo will stay frozen at the fertility clinic (while the biopsy is sent away for testing). The process of freezing the embryo is called vitrification, which avoids the formation of ice crystals. Embryos can be frozen indefinitely, although storage policies differ by clinic.  

An IVF transfer places the frozen embryo in the female partner’s (or surrogate) uterus. In order to do this, medication is generally taken to simulate a menstrual cycle. The embryo would be transferred to the female partner’s uterus 5 days after ovulation. The time commitment for this type of cycle is generally a once-a-week visit to the fertility clinic for bloodwork and ultrasound.  
 
Following the transfer of the frozen embryo, bloodwork is taken about 9 days later (this would be 4 weeks pregnant) to confirm pregnancy. If pregnant, the medication is continued until around 12 weeks pregnant. At this time, care proceeds the same as any other pregnancy.  
 
Unfortunately, not every embryo will lead to a viable pregnancy. After a failed transfer, your provider may recommend diagnostic tests to determine the cause of any issues. Diagnostic tests like a saline-infused sonogram (SIS) or hysteroscopy are common tests to determine if any issues with the lining of the uterus may have caused implantation failure. Another common test looks at endometrial receptivity (ERA) to determine if the embryo is being transferred at the optimal time. Your healthcare provider may also consider a different type of transfer cycle. Not everyone responds the same hormonally to all of the transfer protocols. Adjustments will likely need to be made based on past responses.  

PGT-M can be performed for genetic disorders caused by single genes that are passed from parent to child through one of a few different types of inheritance patterns: 

1. Autosomal dominant disorders
   
2. Autosomal recessive disorders
   
3. X-linked disorders
   

PGT-M is a laboratory test used to identify pathogenic variants in embryos created during the in vitro fertilization (IVF) process. The goal of PGT-M is to identify which embryos contain the pathogenic variant(s) and which embryos do not (click here to watch a brief video about PGT). This sorting tool is used to significantly reduce the chances of transferring an embryo with a specific genetic condition.

A small number of cells are removed (biopsied) from each embryo and sent to a genetic testing laboratory for PGT-M testing.  The cells are taken from a part of the blastocyst called the trophectoderm, this is the part of the embryo that will eventually form the placenta, not the fetus.  These cells are considered representative of the rest of the embryo. The embryos themselves will stay frozen at the fertility clinic while the PGT-M process is completed, which generally takes 2-3 weeks.  A report is sent back to the clinic identifying affected and unaffected embryos. When ready to initiate the pregnancy, an embryo that did not inherit the genetic condition would be selected, thawed, and transferred to the uterus of a reproductive partner or their gestational carrier. 

PGT-M requires the creation of a specific probe or “barcode” to test the embryos for the specific pathogenic variant(s) linked to the disorder of concern while also cataloging specific genetic markers (called single-nucleotide polymorphisms) around the pathogenic variant(s) to maximize the accuracy of the testing. This is performed for both reproductive partners.   

Each PGT-M test is tailor-made for each individual family, using the “barcodes” from each family identified from DNA samples (usually provided via cheek swab) from both partners and frequently additional family members.  

The fertility clinic will review the results from the genetic testing company with you. Embryos will be identified that are suitable for transfer and do not contain the genetic variant(s) of concern. At that time, your provider will address any other issues related to the embryos 

PGT-A testing is a standardized type of testing that looks at the chromosomes on a larger scale. Testing for aneuploidy aims to identify if embryos have the correct number of chromosomes (euploid).  PGT-A can help optimize the IVF process and minimize pregnancy losses due to whole-chromosome abnormalities as the chance of an embryo having too many or too few chromosomes increases with the age of the parents.  The process is faster, more rudimentary, and costs less per embryo. PGT-A does not catch single gene mutations (like PGT-M) or chromosomal rearrangements.  

The genetics laboratory can use the same embryo biopsies to perform both PGT-M and PGT-A testing. Some genetics laboratories perform PGT-M and PGT-A simultaneously, while others allow for sequential testing. 

While the results of PGT-M are highly accurate (>97%), PGT-M is still considered a screening test, meaning it is not diagnostic. It does not diagnose the transferred embryo as having or not having the pathogenic variant(s) that predisposes someone to a genetic disorder. Transferring embryos introduces an element of human error, as well as laboratory error. For these reasons, patients are counseled on the option of minimally invasive diagnostic prenatal tests including chorionic villus sampling (CVS) or amniocentesis to confirm that the fetus does not have the pathogenic variant(s).  Current technology does not have any non-invasive options specific enough to test for single-gene disorders. 
 
Non-invasive prenatal screening (NIPS), which uses a blood sample taken around 10 weeks of pregnancy, is now commonly available as a first-line screen during pregnancy for everyone. It tests for a limited number of whole-chromosome abnormalities during pregnancy. NIPS should confirm the PGT-A findings, but this test will not assess the accuracy of PGT-M.  A healthcare provider can review these options with you in more detail as there are risks and limitations for each test that should be considered carefully.