PGD or Preimplantation Genetic Diagnosis is a diagnostic process used in conjunction with In Vitro Fertilization (IVF). This testing process allows doctors to analyse and screen the embryos to check if a genetic disease or disorder is present. Once the process is completed, the embryo is replanted into the mother’s uterus.
Preimplantation genetic testing has been developed as a reproductive technology that enhances the chances of potential pregnancy and increases the success rate of successful delivery. Preimplantation genetic diagnosis refers specifically to the process where one or both genetic parents have been diagnosed with a genetic abnormality, and the embryo is tested to determine if it has inherited the genetic disorder or abnormality.
Another method called the Preimplantation Genetic Screening (PGS) refers to the techniques through which embryos are analysed for aneuploidy from presumed chromosomally normal genetic parents.
PGD can be considered an additional step in the IVF cycle. However, it proves the most beneficial for patients with an increased risk of chromosome abnormalities or genetic disorders. Patients who should consider PGD testing include women who have undergone multiple miscarriages, experienced chromosomal or genetic abnormality in a previous pregnancy, or tested positive for a genetic disorder.
It has been noted that women over the age of 38 years and men with a sperm-related genetic abnormality can produce embryos carrying such a disorder. If the parents have a structural rearrangement in their genes, the process of PGD can identify the embryos that have the appropriate genetic material.
Preimplantation genetic diagnosis includes three forms of testing. The testing to determine the abnormalities that can result in embryos with genetic disorders is called PGT-A. A in PGT-A refers to aneuploidy. Testing which embryos have a normal number of chromosomes when structural rearrangement is present is called PGT-SR. Here, SR stands for structural rearrangement. If the chances of a child having a genetic disorder is more than 25%, PGT-M is used. In this testing, only those embryos that do not carry any diseases are implanted in the patient. The M in PGT-M stands for monogenic disorders.
In various cases, the process of PGD testing has helped parents ensure that their baby is born without a genetic abnormality or condition that can hamper their child’s life. The testing is used as part of the complete in vitro fertilization cycle. Parents undergoing or planning to undergo the IVF cycle should learn how the PGD test works. The steps below explain the entire process.
All PGD testing and diagnosis cases need to be tailored according to the genetic disease or mutation that can be inherited from the parents or family. Parents will need to undergo genetic and chromosomal testing and provide relevant information to the lab for analysis. The lab will analyse the results and determine if PGD testing is possible.
The lab will then ask for DNA samples from the female, i.e., the egg source, and the male patient, i.e., the sperm source, including additional family members closely related to these sources, such as parents and children. The information and resources collected through these means will help the lab prepare the probe. Based on the genetic disease that the parents carry, this process of probe development can take a few weeks.
After analysing the sources and developing the probe, the specialist doctor or endocrinologist specialising in the reproductive system will prescribe the female patient fertility medication. The medication is provided to simulate the production of eggs in the ovaries. Thus, several eggs are produced at once. If the process of fertilisation begins with a high number of eggs, the chances of the formation of an embryo that does not carry any genetic abnormality increases.
After the stimulation of ovaries and production of excess eggs, these eggs are retrieved. The embryologist at the lab will assess the eggs to determine their health.
Once eggs are analysed by the doctor, the next step will be to fertilise all the eggs in the laboratory. The fertilisation of the egg can be achieved through two methods. The first is when eggs and sperm are allowed to fertilise by natural interaction on a culture dish. Apart from this conventional method, the doctor may also choose to follow a process called intracytoplasmic sperm injection. ICSI is when a specialist injects sperm into an egg. For most PGD testing, the ICSI process is chosen for fertilisation.
Eggs that have been fertilised successfully will be kept in the lab for at least a week. This is required so that they can reach the blastocyst stage. When the embryo reaches this stage, it contains 100 to 150 cells. The specialist at the lab will now conduct a biopsy to remove 2 to 10 cells from the embryo. This is followed by the vitrification of the embryo. The cells removed from the embryo will be sent to the lab for PGD testing.
After receiving the samples of the embryos from the biopsy, the probe developed in the beginning to test will be used to determine if the embryo has inherited the genetic mutation or abnormality. Chromosomal errors can also be tested using the same cells obtained from the biopsy. The results of the test provided by the probe will be transferred to the clinic.
According to the results obtained from the PGD lab, if the patient has healthy embryos that do not carry any genetic condition, the patient can proceed with the transfer of this normal and healthy embryo.
The patient will be asked to take a home pregnancy test or a blood test to determine if they have conceived 12 days after embryo implantation. If the result is positive, the transfer and implantation of the embryo are considered successful.
Patients can discuss which testing procedure will suit them best and how and which embryos will be transferred to the uterus with their doctors or medical teams to make the right decision.
When patients or their close family members have been diagnosed with a genetic disorder, PGD testing is required.
The embryo is tested through the probe developed with the DNA samples of the parents.
The female patient is prescribed fertility medication to increase the production of eggs. The embryologist then retrieves the eggs for lab testing and fertilisation.
Based on the chromosomal structure of the parents, the embryo is tested for relevant chromosomal errors with the help of the probe.