Preimplantation Genetic Diagnosis/Screening (PGD/PGS)
In assisted reproduction techniques, such as in vitro fertilisation (IVF), an embryo is created in a laboratory setting and then transferred into the female uterine. For some couples, particularly those at risk of hereditary disease, questions may arise on the embryos’ genetic health.
What is preimplantation genetic diagnosis?
Preimplantation genetic diagnosis (PGD) is an advanced process during which embryos obtained through an IVF process are tested for genetic abnormalities before they are implanted in the uterine. This is a scientific method to genetically analyse an embryonic cell to detect any known trisomies 13, 18, 21 (Down syndrome), muscular dystrophy (Duecheue Becker), Hemophilia A, B, β thalassaemia, fibrocystic disease, chromosomal translocations, monogenic diseases and determine the gender to prevent the transmission of gender-related diseases to the embryo (fragile X syndrome related with the most common mental retardation forms).
When to apply this
Today, preimplantation genetic diagnosis is clinically applied and it has been integrated in the body of other assisted reproduction techniques. Preimplantation genetic diagnosis is applied with very good results in women with repeated miscarriages due to chromosomal anomalies, women with repeated IVF failures, women with a history of genetic abnormalities in previous pregnancies, couples-carriers of body-related (gender) anomalies.
The procedure at a glance
The procedure includes hormone stimulation of ovaries to create many eggs, which will then be fertilised using IVF and microfertilisation at all times. Preimplantation genetic diagnosis PGD/PGS includes the biopsy of embryos found at the stage of 4-8 cells. Expert embryologists using expert micromanipulation techniques create a hole with laser in the embryonic zona pellucida and then use a special micropipette to aspire one or two cells. Then, the genetic analysis is performed by an expert geneticist. Genetic material from these cells is analyzed thus allowing genetic abnormalities to be detected. Genetic analysis results are given within 24 hours. Based on the results, only those embryos having a normal number of chromosomes or not carrying this particular family’s disease will be selected for transfer and placed in the uterus. Studies show that the rate of abnormal pregnancies following preimplantation genetic diagnosis is five times lower. In all cases, pregenetic testing is required at pregnancies arising after a preimplantation genetic diagnosis.
Biopsy may be performed at:
- Egg level (first polar body biopsy)
- Embryonic level on 4-8 cells (one or two blastomere biopsy)
- Blastocyst level (biopsy on trophectoderm cells).
The predominant technique is a biopsy on 4-8 cell embryos for it provides the highest pregnancy rates.
The biopsy on 4-8 cell embryos and acquisition of up to three blastomeres does not seem to have a negative impact on foetal development or its capability to be implanted.
PGD (preimplantation genetic diagnosis) has been performed since 1989 and ensures the detection of genetic diseases from the embryo as long as it grows under culture conditions at the embryology lab and before it is transferred in the maternal cavity. Up to 200 genetic diseases may be detected. The approximation rate reaches 99.9%.
This method enables couples having an unfavourable medical history (a 25% to 50% chance of having a child with a hereditary disease) to have healthy –not sick- children, as well as avoid the painful duty to terminate the desired pregnancy.