What is preimplantation genetic diagnosis?
The first thing to say about PGD is that it cannot happen in the course of natural conception. PGD can only be carried out with IVF, as the embryo needs to be carefully monitored before, during and after any cells are removed for analysis.
A common reason for doing PGD is where there is a need to prevent a hereditary condition from being transmitted from a parent to the offspring. With PGD, only embryos without the specific mutation can be transferred to the womb.
The steps of PGD
PGD starts with a hormone injection that stimulates the growth and maturation of a much bigger number of eggs than a woman naturally produces every month.
The eggs are then collected in a minor surgical procedure under ultrasound guidance. The male partner provides sperm, which is then used to fertilise the eggs in the embryology laboratory.
Fertilisation can be carried out using standard in vitro fertilisation (IVF), where a large number of sperm are placed within the same laboratory dish as the egg.
Fertilisation can also be performed with the injection of a single sperm cell directly inside the egg (intracytoplasmic sperm injection – ICSI). This avoids contamination of results with DNA from other sperm cells.
Some of the cells of the embryos (normally one to five, depending on the stage of embryo development) are then removed by a procedure called “embryo biopsy” and analysed for the presence or absence of the disorder that runs in the family. If a disease-free embryo is identified, it can then be transferred to the womb.
Even if transferral is successful, implantation (the process through which the embryo attaches to the mother’s womb) depends on other factors and pregnancy is not guaranteed.
While PGD is highly accurate, errors intrinsic to the technique can very rarely occur. As such, it is recommended that the disease-free status is confirmed during pregnancy or after birth.