Pre-implantation genetic testing 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. This new tool 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 (Pre-implantation Genetic Screening – PGS) or for a specific singe gene mutation (Pre-implantation Genetic Diagnosis – PGD).
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.
The term Pre-Implantation Genetic Screening (PGS) is defined by American Society of 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. Downs Syndrome, Turner’s Syndrome, Trisomy 13).
The term Pre-Implantation Genetic Diagnosis (PGD) 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 PGD 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 (PGS or PGD) 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.
PGS for Aneuploidy Screening (abnormal number of chromosomes, related to the chronological aging of eggs)
- Women 35 years or older
- Recurrent 1st trimester pregnancy loss
- History of chromosomally abnormal pregnancy
- Previous IVF failure
- Family balancing of genders
- Improving efficiency of IVF
- Reducing the number of embryos transferred = decreased risk of multiple gestation
PGD for Chromosomal Rearrangement
- Individuals who are known carriers of a chromosomal translocation or inversion
PGD for Single Gene Disorder
- 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
Please Note: 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.
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.
Please click here for an overview of the PGS/PGD process (slideshow presented by Natera)