Embryo preimplantation genetic testing facts
- Preimplantation genetic testing (PGT) screens embryos created through in vitro fertilization (IVF) for genetic and chromosome abnormalities that can cause birth defects, miscarriage or the embryo to not implant in the uterus.
- Embryo preimplantation genetic testing also looks for chromosome problems that can result in conditions in a child like Down syndrome and for single gene defects that can result in cystic fibrosis and other diseases.
- Embryo PGT can also determine genetic gender of the embryo(s) prior to transfer.
- PGT is a new term in fertility medicine that replaces the more familiar terms of preimplantation genetic diagnosis (PGD) and preimplantation genetic screening (PGS), while serving the same functions as those screenings.
- The process involves taking test cells from an embryo as it develops in the lab and evaluating the sample for chromosome disorders or genetic defects, in order to implant only a healthy embryo(s).
- People who should consider PGT include known or suspected carriers of a genetic disorder, older women or those with ovulation problems, couples experiencing recurrent miscarriage, and those who have had previous IVF failures.
What is preimplantation genetic testing (PGT)?
Advances in assisted reproductive technology allow fertility specialists to evaluate embryos created through IVF for genetic health before they are transferred to the mother’s uterus. The best-proven technique for embryo genetic testing is preimplantation genetic testing, or PGT.
Why perform PGT?
- PGT screens for certain genetic conditions that can result in the development of birth defects or even death in a newborn soon after birth.
- PGT also enhances chances of IVF success because genetic abnormalities are also the cause of many cases of implantation failure and miscarriage after infertility treatments.
How is PGT performed?
Preimplantation genetic testing involves performing tiny biopsies on 5- or 6-day old embryos created through IVF. These samples are evaluated by a genetics lab to identify embryos with chromosomal abnormalities or specific gene defects.
This allows a fertility specialist to select only genetically healthy embryos to use in the embryo transfer part of IVF in which embryos are placed in the woman’s uterus for implantation.
Data show higher than a 65% pregnancy rate using PGT, which is far superior to earlier approaches to embryo genetic testing and IVF without genetic testing. As PGT technology keeps improving, it is becoming more accurate and can identify subtle aspects of genetic abnormalities in embryos.
Why do genes matter in fertility medicine?
Genes, the DNA sequences that serve as a roadmap to a person’s growth, are positioned on chromosomes. Every human has two copies of each chromosome, one inherited from the mother and one inherited from the father, for 23 chromosome pairs.
Genetic diseases, traits and disorders, known as recessive conditions, are passed from parent to child as a change or mutation in a pair of chromosomes. Genetic recessive conditions include Down syndrome, cystic fibrosis or sickle cell anemia.
If a person carries only one recessive gene within a pair of chromosomes, it means she or he does not suffer from the disease but can genetically pass it on to a child. These people are called carriers. If two parents (or donors) are each carriers of the same recessive condition, each can pass along one gene to the child. That child will inherit a pair and have a 1 in 4 chance of being affected by the condition.
Common questions about preimplantation genetic testing
How does PGT improve IVF success?
PGT identifies genetically normal embryos to use for implementation during an embryo transfer, which greatly affects the success of embryo implantation and carrying the pregnancy to term.
Does PGT cost extra?
Preimplantation genetic testing is an additional cost to the cost of IVF treatment. However, adding PGT may also save money by increasing the chances of success in a single IVF cycle, eliminating the cost of multiple embryo transfers or miscarriage.
Can PGT help in family balancing?
PGT can also reveal gender information, and parents can select the gender of the healthy embryo they wish to have implanted.
PGT is the new name for PGD and PGS embryo genetic testing
Preimplantation genetic testing is a change in how fertility medicine refers to the previously called PGD (preimplantation genetic diagnosis) and PGS (preimplantation genetic screening). PGD looks for a specific defective gene, and PGS looks for abnormalities in chromosome-pair number.
PGT evaluates for the same things in the following three types of screenings.
PGT-M (single genetic/monogenic)
PGT-M tests for a single genetic (monogenic) defect or mutation, like cystic fibrosis, BRCA1, muscular dystrophy, Huntington’s disease and Fragile-X syndrome.
PGT-A (abnormal chromosome pair number)
PGT-A tests for the presence of an abnormal number of chromosomes in a chromosome pair, either one less or one extra (called aneuploidy). Aneuploidy can cause conditions such as trisomy 21, or Down syndrome, and is a strong indicator for birth defects or miscarriage.
PGT-SR (structural rearrangements)
PGT-SR tests for chromosome structure rearrangements, which often cause recurrent miscarriage and embryos with improper chromosomes, and also greatly decreases the chance of live birth.
Genetic issues we test for
If both parents carry a recessive genetic disorder, there is a 25% chance their child will have the disorder and a 50% chance the child will be a carrier. Without testing, parents may not know they are at risk of passing a devastating genetic disorder to their children.
An estimated 2.5 million Americans carry the sickle cell trait. This disease forces the body to create pointed, hard red blood cells that block normal blood flow to key organs.
Screening for spinal muscular atrophy
Spinal muscular atrophy (SMA) is a group of inherited genetic disorders that result in progressive muscle wasting. It is one of the most common genetic conditions affecting children, with an estimated 1 in every 6,000 babies born with SMA worldwide each year.
One of the most common inherited genetic disorders, cystic fibrosis forms mucus build-up in the pancreas and lungs, restricting breathing and causing frequent lung infections.
This is the evaluation of embryos to determine if they have the correct number of chromosomes or an improper arrangement of chromosomes. Down syndrome is one possible result of an aneuploid embryo.
Tay-Sachs is just one among many genetic diseases common in the Ashkenazi Jewish population. One out of three Ashkenazi Jews in this country carry at least one Jewish genetic disorder.
Is PGT right for me?The following couples or individuals can benefit from preimplantation genetic testing:
- Individuals or couples with known genetic conditions or a family history of genetic conditions.
- Women over 35 or those with ovulation difficulties.
- Couples with previous recurrent miscarriages.
- Couples with previous unsuccessful infertility treatments.
- Couples with an unexplained infertility diagnosis.
- As with many tests, PGT may not be 100% accurate as it is considered a screening test, not a diagnostic test. There is still a chance of implanting an embryo with a disorder.
- A pregnant woman who underwent PGT because she was considered at risk of a flawed embryo should still have prenatal genetic testing done.
- The biopsy, handling, freezing and thawing result in damage to about 5% of embryos, causing implantation failure.