PGD is based on the principle of distinguishing between embryos with a disorder on the one hand and embryos without a disorder on the other, then implanting only those embryos which do not carry the disorder in the female’s uterus. PGD involves screening embryonic cells. Obtaining these embryos requires an IVF or ICSI procedure. For a successful PGD procedure, at least four or eight follicles must ripen, depending on the indication.
Once an ovum (egg cell) has been fertilised in the lab, it will start to divide. Once an ovum has divided, it is called an embryo. By the third day following fertilisation, an embryo generally consists of eight cells. At this stage, one or sometimes two cells of the eight-cell embryo will be removed in a process which is called a biopsy. A very thin needle or laser beam will be used to create a tiny opening in the membrane covering the ovum. A slightly larger needle will then be used to remove one or two cells from the embryo. The removed cells will be examined at the laboratory of Maastricht UMC+’s Clinical Genetics unit.
Implanting ‘healthy’ embryos
If the results of the tests of the removed cell indicate that it does not show any signs of the disorder for which it was being screened, it can be assumed that the embryo from which the cell was removed is free from this particular disorder. This genetic test is completed within one day, thus allowing the ‘healthy’ embryos to be implanted in the woman’s uterus on the fourth or fifth day following follicular puncture. A PGD procedure involves implanting one embryo in the female’s uterus. If any suitable embryos are left after that, the doctors may decide to cryopreserve these for future implantation, provided that the embryos satisfy certain quality requirements.
Two embryos may be implanted simultaneously in women aged 38 or older, or in those who are on their third treatment cycle.