Preimplantation Genetic Diagnosis - Overview, Indications and Conditions, Process - Pre-implantation diagnosis by embryo biopsy

In short: Preimplantation genetic diagnosis means taking a small biopsy from the embryo in the laboratory to test it for inherited disease before transfer, so that only a healthy embryo is placed in the uterus. It is now done at the blastocyst stage (day 5 or 6) and can detect conditions such as thalassaemia, sickle-cell anaemia, haemophilia and muscular dystrophy.

Genetic diseases are considered to be a major problem. Concentrated efforts have been done by scientists to solve the problem. We will mention a few of the steps taken.

Pre-implantation diagnosis by embryo biopsy:

This is done by taking one or more cells from the embryo. The embryologist removes only a few cells — as few as needed for an accurate diagnosis — so as not to harm the embryo. Today the preferred time is the blastocyst stage, on day 5 or 6 after fertilization, when a few cells are taken from the outer layer that will form the placenta (trophectoderm biopsy); biopsy at the 8-cell stage on day 3 is now rarely used. The lab technician creates a hole in the embryo wall by using a very thin microscopic needle or chemical substance, or laser. A few trophectoderm cells (the outer cells that will form the placenta) are then removed, without taking cells from the inner cell mass that forms the baby. After that, the diagnosis is done in 2 ways:

If you would like to understand how eggs are fertilized in the laboratory first, our guide to the IVF process walks through each stage.

2. FISH (fluorescence in situ hybridization):

In this technique, the normal embryos can be selected to be transferred to the uterus as in the case of IVF. It should be taken in to consideration to select the best time to take the biopsy from the embryo and to avoid occurrence of damage to the embryo. In that way, we can achieve the best results in diagnosis which will lead to a healthy, successful pregnancy.

1. Genetic diagnosis of pre-implantation embryo:

This technique is used to diagnose the inherited diseases and has been developed recently, by using a special test called PCR (Polymerase Chain Reaction) and the number of human genetic diseases that can be diagnosed is increased. We hope, in the future, scientists will be able to diagnose all the genetic diseases in that way. This technique is useful for couples that may have a child who is effected with an inherited disease. In that way, only the normal embryos can be transferred to the uterus. This is much better than the occurrence of a pregnancy than the diagnosis of a genetic disease by Amniocentesis (taking sample of the amniotic fluid), or by Chorionic villous sampling.

Because no test is 100% accurate, confirmation of the result during pregnancy by chorionic villus sampling or amniocentesis is still recommended after PGT.

Gene disorders transmittable to the offspring, which can be analysed by genetic diagnosis after oocyte and embryo biopsy.

List of some of the genetic diseases that can be diagnosed by PGD

2. FISH (fluorescence in situ hybridization):

FISH was used in the past to check embryo chromosomes, but it could only test a few chromosomes and has now been replaced by modern methods such as next-generation sequencing (NGS), which examine all chromosomes; these are used for couples who carry a chromosome rearrangement (PGT-SR) and, in selected cases, for chromosome screening (PGT-A). It should be taken in to consideration to select the best time to take the biopsy from the embryo, and to avoid the occurrence of damage to the embryo. We can then hope for the best results in diagnosis, leading to a healthy and successful pregnancy.

Dr Najeeb Layyous F.R.C.O.G

Consultant Obstetrician, Gynecologist and Infertility Specialist