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 reaches eight-cell stage, one or two cells are removed from the embryo, or it can also be done by removing the trophectoderm cells from the blastocyst. Genetic analysis is then perfomed on the cells that is obtained, to see whether the embryo contains the abnormal gene or chromosome. The analysia is done using polymerase chain reaction (PCR) and flourescence in situ hybridization (FISH). PCR is done to amplify the DNA fragment specific sequence, while FISH is used to detect chromosomal abnormalities.
The remainder cell mass from the embryos with normal gene will be transferred back into the uterus and allowed to implant and developed. Those embryos 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 fertilisation.