Preconception Genetic Testing
Preconception genetic testing
The different types of testing
Preconception genetic testing is testing which takes place before a couple decides to conceive. There are two main types of preconception genetic testing- carrier screening and preimplantation screening.
The aim of the carrier testing is to understand more about the genetic health of the parents, and what this might mean for the health of their children-to-be in the context of rare diseases.
Pre-implementation testing tests the genetic health of the embryo before it is implanted in the mother, and identifies any potential rare diseases or the genetic changes that might cause one.
Genetic carrier screening is genetic testing performed on potential parents-to-be before a pregnancy. The testing aims to understand more about the genetic health of the parents-to-be being tested, in order to understand the risk of passing on a gene mutation, and potentially a rare disease to their future children.
Expanded carrier screening tests for over 100 rare diseases, and this number grows all the time. A parent who is a carrier will not necessarily pass the gene mutation on to their children, but their children’s risk for developing a rare syndrome from that gene change increases, depending on the type of inheritance pattern involved. Some syndromes require that both parents pass on the gene or chromosome change for a child to be affected or at higher risk of being affected.
The limitations of carrier screening include the fact that it screens for just 100 syndromes, when there are currently over 7000 rare diseases identified. It can not catch everything. It also can only catch those syndromes that are inherited- many syndromes are caused by new or spontaneous gene or chromosomal changes that occur during the reproductive process. Carrier screening is not able to identify these, or even identify a greater risk of developing them.
This type of preconception genetic testing or screening involves testing embryos before they are transferred into a uterus, as a part of IVF treatment. It involves taking a biopsy, a few cells, from the embryo during the blastocyst stage. This can be anywhere between days 5 and 7 of the embryo’s development. These cells are removed from the outer layer of the embryo, a part of it which will eventually develop into the placenta. These cells are sent for genetic testing, a process which may take anywhere from 10-14 days. The embryo is then frozen until the test results are ready.
This type of preconception genetic testing (PGT-A) is able to identify if an embryo is at higher risk of not developing into a healthy pregnancy, or at higher risk for ending in miscarriage. In terms of genetic testing it is also able to identify missing or extra chromosomes in the embryo. These may result in higher risk of miscarriage, or in the development of a rare disease. This type of testing is generally recommended for women of an older age, or with a known lower egg quality. For this reason preimplantation testing is generally recommended for women over 35 years of age.
An embryo identified with chromosomal abnormalities that may prevent it developing into a pregnancy, or may put it at risk of miscarrying, is known as an aneuploid. It may also, in some rare cases, mean the development of a baby with a genetic disorder.
Another type of pre implantation testing or screening is known as PGD or PGT-M. This is testing to identify specific genetic syndromes or rare diseases in an embryo. It can identify single-gene disorders, which means both parents are most likely to be carriers for the same genetic mutations.
PGD testing will be recommended to parents-to-be who know they are carriers, or who have a child already born with the genetic syndrome. It is not blanket screening for any and all rare diseases, and genetic conditions, it is testing for known conditions and is very targeted screening. It tests for a very specific genetic change that is known to those testing for it.
Before beginning any form of preconception genetic testing, it is important for parents-to-be to attend genetic counseling services.
A genetic counselor will be able to explain each of the testing options in detail, including the process of each, and they will also be able to explain all of the possible test results and the next steps based on each.
For carrier screening, depending on how a gene mutation is inherited, and if it requires inheritance from one or both parents as carriers, will determine a child’s risk for developing the same mutation and a rare disease from it.
Genetic counselors help parents understand every part of this process, and the potential risk for their future children. This in turn helps parents make informed decisions.
Genetic testing also offers important emotional and counseling support throughout the testing and diagnosis process. This is important for many people during what can be a very difficult and stressful time.
Genetic counseling is an essential part of the preconception genetic testing process and should not be skipped.