Preimplantation genetic diagnosis (PGD) is a technique used to identify genetic defects in embryos formed before transferring them back into the uterus. It requires IVF procedure to obtain embryos for PGD evaluation. PGD will help the couples at risk of an inherited disorder or mutation gene to prevent the transfer of affected embryos into the uterus. This is to avoid selective pregnancy termination of the affected child. It is considered as an alternative to prenatal screening.
PGD can also help those patients who undergo IVF treatment to screen the embryos for chromosome aneuploidies. In this situation, PGD is not used to perform specific prenatal diagnosis, but instead it used to screen the embryos for chromosomal abnormalities. It is better referred to as preimplantation genetic screening (PGS). The main indications for PGS include advanced maternal age, history of recurrent miscarriages, patients who have given birth to genetically abnormal babies. It has also recommended for patients with obstructive and non-obstructive azoospermia because there is increased risk of foetal abnormalities.
Diseases that can be detected through PGD include:
- Down’s syndrome (trisomy 21)
- Edward’s syndrome (trisomy 18)
- Patau’s syndrome (trisomy 13)
- Klinefelter’s syndrome (47, XXY)
- Cystic fibrosis
- Tay-sachs disease
- Huntington’s disease
- Sickle-cell disease
- Fragile-X syndrome
- Haemophilia A, etc
PGD is done when an embryo becomes a blastocyst which normally has more than 100 cells five days after fertilization, three or more cells are biopsied from the blastocyst, or it can also be done by taking one or two cells from an 8-cells embryo. Genetic analysis is then perfomed on the cells that is obtained, to see whether the embryo contains the abnormal gene or chromosome. The analysis is done using Next Generation Sequencing (NGS). NGS is a new technology for comprehensive chromosome testing of IVF embryos or blastocyst. NGS for PGD is replacing other technologies as the predominant method for preimplantation genetic screening (PGS) due to lower cost and reduced errors. In fact, MetroIVF is one of the earliest to use this advance technology.
The remainder cell mass from the embryos or blastocyst with normal gene will be transferred back into the uterus and allowed to implant and developed. Those embryos or blastocyst that are chromosomally or genetically abnormal will not be transferred and allowed to perish. Normally the genetically normal embryos will be cultured until blastocyst stage and transferred back into the uterus five days after fertilization.