Preimplantation genetic testing overview
- Preimplantation genetic testing (PGT) of embryos has made tremendous advances since its introduction in 2005. Although genetic testing could be performed on human cells before 2005, techniques were developed to perform genetic testing on just a few cells removed from a developing embryo.
- Genetic testing allows prospective parent(s) undergoing in vitro fertilization (IVF) to test for a specific gene or for chromosomal copy number abnormalities before transferring the embryo into the uterus. This can increase pregnancy rates and decrease miscarriage rates.
- Chromosomes are structures found in the center of the cell nucleus. A human typically has 23 pairs of chromosomes, for a total of 46 individual chromosomes. An embryo receives 23 chromosomes from the sperm and 23 from the egg. Chromosomes are made up of genes, which contain the genetic information that instructs the body on how to function.
- Embryos can be tested prior to an embryo transfer for their chromosomal copy number or rearrangement (preimplantation genetic testing – PGT) or for a specific singe gene mutation (PGT-M).
- This testing is performed on cells from an embryo biopsy on the 5th or 6th day of embryo development. Embryos are frozen, or “banked,” until results of the genetic testing are available.
- Embryos that are identified to have the correct number of chromosomes (euploid) and/or do not carry the specific mutation tested for (“unaffected”) can then be transferred into the patient in a separate FET (frozen embryo transfer) cycle.
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The term preimplantation genetic testing (PGT – formerly known as preimplantation genetic screening and diagnosis, PGD and PGS) is defined by American Society for Reproductive Medicine (ASRM) as testing performed to screen embryos for extra or missing chromosomes when the genetic parents are known or presumed to be chromosomally normal.
Having an extra or missing chromosome(s) (called aneuploidy) can result in lack of implantation of an embryo, pregnancy loss, and children born with chromosomal abnormalities (i.e. Down syndrome, Turner syndrome, trisomy 13).
PGT-M (preimplantation genetic testing for monogenic/single gene mutation) applies to testing performed to determine whether a specific genetic mutation or chromosome abnormality has been transferred to the embryo when the genetic parents are known to have that specific genetic mutation or a chromosome abnormality.
The purpose of PGT-M is to identify an unaffected embryo for transfer, thereby eliminating the chance that a child would be affected by the known disease or abnormality.
Genetic testing for embryos
Genetic testing is not necessarily a recommended component for all patients undergoing IVF, although the frequency of its use is rapidly increasing.
The following outlines some of the common reasons a patient/couple might choose to genetically test embryos.
PGT for aneuploidy screening
Aneuploidy is an abnormal number of chromosomes in embryos, related to the chronological aging of eggs.
Candidates for aneuploidy screening:
- Women 35 years or older
- Recurrent first trimester pregnancy loss
- History of chromosomally abnormal pregnancy
- Previous in vitro fertilization (IVF) failure
- Family balancing of genders
- Improving efficiency of IVF
- Reducing the number of embryos transferred = decreased risk of multiple gestation
PGT-M for chromosomal rearrangement
Candidates for chromosomal rearrangement include individuals who are known carriers of a chromosomal translocation or inversion.
PGT-M for single gene disorder
Candidates for single gene disorder PGT-M include individuals who are known carriers of a specific genetic disorder (such as cystic fibrosis or Tay-Sachs), to identify embryos that are either an affected or carriers of the mutation.
PGT-M for BRCA gene (breast cancer gene)
The BRCA gene is commonly known as the “breast cancer gene.” Women who carry this gene are known to have a 65% chance of being diagnosed with breast cancer and a 45% chance of developing ovarian cancer by the age of 70.
Many women (including high profile celebrities such as Angelina Jolie) opt to have a double mastectomy and/or hysteroscopy to reduce the chances of cancer. Thanks to the technology of preimplantation genetic testing women who carry this gene can pursue IVF with PGT-M and ensure that their children will not be carriers of this gene.
A chromosomally tested embryo will not always result in a pregnancy. Factors other than those tested (ie, specific genes), the metabolics of the embryo and the uterine environment all affect the pregnancy outcome.
Healthy baby: prenatal screening after genetic screening
A “healthy” baby is the result of chromosomal normality, genetic normality (not currently able to test all genes) and non-genetic factors (cleft lip, heart defects) that are affected by environmental factors, epigenetic changes, and other unknowns.
These are best screened for with good prenatal care and ultrasound. It is never possible to guarantee a “healthy baby,” even with genetically tested embryos.
All newborns carry a risk of 3-5% of having a congenital abnormality, and not every condition that can affect a baby can be tested for. It is important that you follow the recommendations of your obstetrician regarding prenatal and newborn screening.