Genetic Testing and Diagnosis
Pre-Implantation Genetic Diagnosis (PGD) and Pre-Implantation Genetic Screening (PGS)
Ongoing advances in genetic science have enabled us to identify many of the genes responsible for physical traits, including inherited disease. This has in turn enabled new technologies to screen embryos and identify those that may be optimal for implantation.
Two different procedures are available to those who want to test embryos for genetic disease: Pre-Implantation Genetic Diagnosis (PGD), in which embryos are genetically analyzed for specific mutations before being implanted into the woman’s uterus; and Pre-Implantation Genetic Screening (PGS), in which each embryo is tested for chromosomal numbers prior to implantation.
While PGD is not necessary or even advised for everyone, it is an invaluable technology for those who may be carriers for a genetic disease, as it can greatly increase the chance of a healthy offspring for these patients.
Those who choose to have embryos diagnosed must first undergo IVF (in vitro fertilization), in order to produce sufficient embryos and embryonic cells to test. In IVF, the female partner receives fertility medications, which stimulate her ovaries to produce multiple eggs. Eggs are removed and fertilized with the male sperm, in our laboratory, and early embryos (blastocysts) begin to form. Learn more about IVF.
Ideally, the couple undergoing IVF will produce multiple embryos. At day 5 of embryonic development, a small cellular sample is removed (biopsied) from each embryo and sent to a highly specialized pre-implantation genetics laboratory, where it is tested for any mutations. This process does not harm the embryos, which continue to grow as they otherwise would. After testing, embryos determined to be free of genetic disease may be transferred into the woman’s uterus. Because the testing process takes several days, embryos may need to be frozen. In this event, a frozen embryo transfer cycle is arranged once PGD results are known.
When is PGD recommended?
PGD is recommended only for those individuals or couples who have or carry a specific genetic disease and are thus at risk for passing this condition on to their child.
There are various ways to determine that a male or female partner may carry a genetic abnormality: a couple may have previously given birth to a child with a birth defect or serious medical condition; a family member(s) may carry a genetic mutation; or a condition may be found during routine, pre-treatment genetic screening -- which is why we screen almost everyone for the common genetic disease Cystic Fibrosis. In addition, a patient’s ethnic background may be associated with certain genetic abnormalities, and thus increase their risk of being a carrier. For this reason, we screen patients for Sickle Cell anemia, thalassemias, and diseases such as Tay-Sachs, if appropriate to do so, based on ethnicity.
Once a genetic abnormality has been identified, the next step is to work with our genetic counselor or a counselor of a patient’s choosing to determine whether PGD is available for their known condition.
Additional Genetic Testing During Pregnancy. The accuracy of PGD is generally between 95% and 99%. Those wanting to confirm a PGD diagnosis are advised to undergo additional genetic testing during early pregnancy, which will confirm the pre-implantation genetic diagnosis to near 100% accuracy.
The procedures available to test for genetic defects during pregnancy include amniocentesis and CVS (chorionic villus sampling).
Amniocentesis. Amniocentesis may be conducted between 16-20 weeks of pregnancy. Here, a fine needle is inserted into the woman’s abdomen and a small sample of amniotic fluid is taken from the uterus. The sample contains cells from the growing fetus, and these can be analyzed for genetic abnormalities.
CVS (chorionic villus sampling). This procedure may be conducted earlier than amniocentesis, between 10-12 weeks of pregnancy. Here, doctors remove a tiny sample of the placenta via a small needle inserted into the abdomen or a small tube inserted through the cervix. As with amniotic fluid, this embryonic tissue sample can reveal genetic abnormalities.
Because amniocentesis and CVS are conducted fairly early in pregnancy, one may know the health status of their pregnancy and make the choice that is right for them, that is, whether or not to terminate the pregnancy.
The risk of losing the pregnancy due to CVS or amniocentesis is about 2%.
Disorders for which PGD is offered. PGD is capable of analyzing the majority of genetic diseases and is an accessible technology for most patients. Access a comprehensive list of the diseases for which PGD is available here.
Those undergoing IVF may also have embryos tested through a process called Pre-implantation Genetic Screening (PGS). Here, embryos are analyzed for the number of chromosomes each contains. Chromosome count is an important indication of embryo health. Too few or too many chromosomes may indicate a number of problematic conditions, including early miscarriage and Down Syndrome, a genetic condition marked by significant physical and cognitive difficulties.
When is PGS recommended?
PGS may be recommended to women who have experienced recurrent miscarriage or repeated unexplained IVF embryo implantation failure. We may also recommend PGS to those becoming pregnant after the age of 35, when chromosomal defects are more likely to occur. The screening is generally available to those who produce six or more embryos through IVF.
PGS can also enable parents to identify the sex of the embryos, as some may wish to transfer embryos of a given sex. Sex selection is often chosen by parents that already have a child or children of the other sex. This is then referred to as “family balancing.” We at PFC do not necessarily advocate undergoing IVF solely for Sex Selection but we do understand that there may be specific personal reasons for this choice and we do offer it to those who wish to do it. Those who make this decision are advised that they may be eliminating roughly half of their embryos from consideration for transfer, and thus sex selection may diminish their overall chances of achieving a pregnancy.
Those who decide to pursue PGS are asked to meet with a genetic counselor at our Northern California San Francisco Bay Area fertility clinic prior to their cycle. This is so we are able to ensure that they have a complete understanding of the genetic evaluation process and its potential ramifications.
As part of the PGS process, parents are required to provide a blood or cheek swab sample to our genetic lab. This allows the PGD/PGS lab to prepare and validate the samples the lab will later receive from their embryos.