PGS & PGD Genetic Testing
PGS & PGD genetic testing at a glance
- Preimplantation genetic testing evaluates an embryo for genetic abnormalities or inherited diseases prior to being implanted into a woman during in vitro fertilization (IVF).
- Preimplantation genetic screening (PGS) tests an embryo for a variety of chromosomal abnormalities, which are linked to failed pregnancies and genetic disorders such as Trisomy 21 (Down syndrome).
- Preimplantation genetic diagnosis (PGD) tests an embryo for a single, specific genetic disorder either parent may carry or be suspected of carrying, such as Huntington’s disease or cystic fibrosis.
- The goal of preimplantation genetic testing is to ensure that embryos with the highest chance of achieving a healthy pregnancy and child are used during IVF.
What is preimplantation genetic screening and diagnosis?
Preimplantation genetic testing, or embryo biopsy, are terms referring to techniques that identify genetic or chromosomal defects in embryos prior to using them in IVF. There are two types of genetic tests available for embryo evaluation: preimplantation genetic screening (PGS) and preimplantation genetic diagnosis (PGD).
PGS tests broadly for multiple genetic problems or chromosome abnormalities (which affect 25-28 percent of embryos, depending upon a woman’s age). In addition to playing a part in some diseases, research shows that chromosome abnormalities can increase the chance of miscarriage and failed IVF.
PGD, on the other hand, is a diagnostic test that searches for a specific genetic disease known to be present in one or both partners’ families.
These tests are conducted in order to check for genetically normal embryos. The goal of Advanced Reproductive Medicine’s genetic testing is to implant the fewest embryos possible that have the highest chance of getting the patient pregnant and of her delivering a healthy child.
For couples with a known risk of heritable, debilitating diseases, genetic testing can lower the risk of passing that disorder on to their child. Genetic testing can also help increase the chances of a healthy pregnancy by screening for potential genetic problems that can negatively affect pregnancy and birth.
What is PGS used for?
PGS is commonly used to evaluate embryos, via cell biopsy and testing, for a normal amount of chromosomes (called euploidy). A normal embryo is comprised of 23 chromosomes inherited from both parents during fertilization (for a total of 46). An abnormal amount, either too many or two few, is called aneuploidy and is a major cause of birth defects and failed pregnancies.
Common conditions caused by chromosome abnormalities include:
- Down syndrome
- Turner syndrome
- Klinefelter syndrome.
Individuals and couples should consider PGS if they are known to carry the chromosomal abnormalities that cause the conditions above or are suspected of carrying them, which is generally determined through detailed family history evaluation by a trained geneticist. The information provided by PGS may help couples and individuals achieve a baby free of birth defects due to chromosomal abnormalities. Additional benefits of PGS include:
- Decrease the possibility of miscarriage
- Minimize the risk of abnormal pregnancy
- Improve chances of a successful birth
- Increase successful pregnancy odds using elective single embryo transfers (eSET), which pretty much eliminates the chance of a multiples (twins or more) and the accompanying health risks to mother and child.
- Reduce the amount of time and costs involved with multiple IVF cycles
- Increase the odds of woman age 37 or older having a successful pregnancy, as age increases the odds of chromosome abnormalities in an embryo.
What is PGD used for?
Many diseases are the result of specific gene mutations in DNA that embryologists can detect through PGD. Individuals and couples who are known carriers of a specific genetic disorder, or have a family history of one, have an increased risk of having a child with that genetic disorder.
People don’t have to possess the genetic defect in order to pass it on to their child. That’s why family history is an important indicator of a potential genetic issue. A geneticist performs a detailed family history evaluation of couples or individuals in order to identify problems in their family tree, and may recommend PGD accordingly.
PGD can identify embryos created during IVF that have such genetic defects. Those embryos are not chosen for implantation, and only embryos free of genetic disorders are implanted. In addition to ensuring a child born without the specific genetic mutation, implanting a defect-free embryo reduces the chances of a failed pregnancy.
The most frequently diagnosed diseases and disorders found through PGD are:
- BRAC 1 & BRAC 2 genetic mutations that predispose women to breast and ovarian cancers
- Cystic fibrosis
- Duchenne muscular dystrophy
- Fragile X syndrome
- Hemophilia A
- Huntington’s disease
- Myotonic dystrophy
- Sickle cell anemia
- Spinal muscular atrophy
- Tay-Sachs disease
Couples or individuals concerned that they may have a genetic mutation that causes one of the above conditions should talk to their fertility physician about possible PGD. Other reasons people may consider PGD include already having a child with a genetic problem and women with unexplained infertility or who have had two or more miscarriages.
Risks and concerns with preimplantation genetic testing (PGS & PGD)
Health risks to children born after PGD or PGS testing are the same as for those children born through regular IVF treatment, and are only slightly higher than for babies born through conventional pregnancies. These risks include low birth weight and increased chance of multiple births (twins or more).
Some embryos may be damaged during cell removal for either PGD or PGS testing. This risk is extremely low and could result in an embryo ceasing development. Otherwise, the medical risks of genetic testing are the same as for IVF.
PGD and PGS involve ethical issues individuals may have strong opinions about. These issues affect couples differently, and we recommend speaking with one of Advanced Reproductive Medicine’s on-staff psychologists prior to PGS or PGD. These concerns include:
- An embryo can have a genetic mutation or a chromosomal abnormality but it may never manifest itself in a disease during the child’s life, making the decision to discard the embryo a difficult one.
- PGD can evaluate an embryo for a specific gene that indicates the resulting child (called a “savior sibling”) could be a match for a sibling in need of a transplant, such as of bone marrow, raising ethical concerns of putting the savior sibling at risk during the medical procedure.