PGS: Preimplantation genetic screening
Preimplantation genetic screening is a method allowing to choose the embryo with normal number of chromosomes for transfer to the uterus during IVF cycle. Embryo with correct chromosomal constitution has got better chance to implant and not to be miscarried.
Normal chromosomal constitution
Each cell of healthy person contains 22 pairs of chromosomes (one pair from the mother and the other from the father) and two sex chromosomes that determine the sex of the individual.
What is aneuploidies
Not all the embryos contain correct number of chromosomes. If there is chromosomes missing or on the contrary there is some chromosomes extra such a condition of embryo is called aneuploidies.
What happens to embryo with aneuploidies
Usually the pregnancy with aneuploid embryo ends up in spontaneous miscarriage during the first trimester.
However, some types of aneuploidy in embryos may lead to the birth of affected child, e.g. Down syndrome (extra chromosome 21), Edward’s syndrome (extra chromosome 18) or Patau’s syndrome (extra chromosome 13).
The risk of aneuploidy in embryos increases with higher age of women and is probably the main reason for sharp decline in a woman’s fertility during the third and early fourth decade of life.
Why to do PGS
The purpose of PGS is to choose healthy embryo rather than aneuploid embryo to be transferred to mother’s uterus. Consequently, the method allows to maximise the chance of giving birth to a healthy child and a successful IVF cycle.
Who is PGS recommended for
- women in late thirties early forties
- women with recurrent spontaneous miscarriages
- previous unsuccessful IVF cycles
- miscarriage or delivery of foetus with chromosomal abnormality
- chromosomal abnormalities detected in a couple
- using sperm after TESE, MESA
How does PGS work
PGS is always performed as part of the IVF process. It means that female partner takes hormonal medications to stimulate the ovaries to produce multiple eggs. Eggs are retrieved by transvaginal collection and after that are fertilised by ICSI with partner’s sperm.
Genetic material can be obtained by biopsy from embryos which are 3 or 5 or 6 days old. One cell is taken from three days old embryos whereas the group of cells is biopsied from 5 or 6 days old embryos.
All embryos are vitrified (cryopreserved) after the biopsy and stored in embryological lab until the results from genetic lab are obtained.
What methods are used for preimplantation genetic screening
At the moment there are two genetic methods available: Array-CGH (a-CGH) microarray technology and NGS (next generation sequencing). Both methods allow the testing of the entire spectrum of chromosomes, 23 pairs. NGS has even greater sensitivity and is even able to detect embryo mosaicism.