Preimplantation Genetic Diagnosis: how does it work?
Last Reviewed 1/09/2016
PGD is a technique that enables couples with a particular inherited condition in their family to avoid passing it on to their children. The process helps potential parents prevent the birth of a child with a serious genetic condition. The procedure for PGD is similar to that of in vitro fertilisation (IVF), with an extra step to check whether embryos are affected by a serious genetic condition.
The typical treatment cycle for PGD is as follows:
Step 1: Stimulation of the ovaries
In order to create embryos for genetic testing, the ovaries have to be artificially stimulated using hormones to produce several eggs at the same time.
Because a significant number of a couple’s embryos are likely to be affected by the genetic or chromosomal abnormality, a large number of embryos need to be created for the best chances of success.
Step 2: Collection of eggs from the ovaries
At the appropriate time, the eggs are removed in during a surgical procedure known as egg retrieval. Once the eggs are removed, they are inspected to determine which are mature and normal appearing.
Step 3: Insemination/injection of sperm
Fertilisation during PGD will be done either by:
- In Vitro Fertilisation (IVF): Sperm and eggs are placed together in a culture dish to allow fertilisation to occur.
- Intracytoplasmic Sperm Injection (ICSI): A single sperm is injected in the centre of an egg. This technique is used for conditions caused by a single faulty gene.
Step 4: Fertilisation
The morning after injection/insemination of the sperm, the embryologist carefully examines each egg to see if fertilisation has occurred.
Step 5: Embryo biopsy
The eggs that were successfully fertilised are grown in the laboratory for 5-6 days when the embryo forms what is called a blastocyst of about 100-150 cells. At this stage trophectoderm biopsy (the cells which will form the placenta) takes place.
This technique is highly specialised and can only be carried out by embryologists who have a special licence from the Human Fertilisation and Embryology Authority (HFEA).
The removal of these cells should not harm the embryo.
Step 6: Embryo Testing
The cells are tested to see if the embryo from which they were removed contains the abnormality that causes the genetic condition.
Step 7: Embryo Transfer
Only those embryos that do not have the specific genetic condition that was tested for will be transplanted into the woman’s uterus. Usually one embryo will be transferred to the uterus at any one time to avoid the possibility of multiple births (more than one baby in a pregnancy).
Any suitable remaining unaffected embryos will be frozen for later use.
Those embryos that are affected by the inherited condition are allowed to perish or couples are asked if they would consider allowing these embryos to be donated for research and training.
Step 8: Pregnancy Test
Twelve days after the embryo transfer, the woman is given a pregnancy test. A positive pregnancy test means that an embryo has implanted.
Would preimplantation genetic diagnosis guarantee the birth of a child without my condition?
Accuracy of testing
The accuracy of PGD will vary, and there is the possibility that testing may not be 100% reliable or conclusive. However, testing is 98-99% accurate for most couples. The risks are dependent upon the condition for which PGD is being offered and the couple’s individual test results.
All patients are offered confirmatory prenatal testing (amniocentesis or chorionic villus sampling) if PGD results in a pregnancy.
Chances of conception
It is difficult to assess success rates for PGD because there is currently little data available. As with most fertility treatments, success depends on many factors, including the woman’s age and weight.
Additionally, sometimes no embryos are suitable for transfer to the womb, for reasons including:
- Not enough eggs were produced or fertilised
- Embryos do not develop to the balstocyst stage
- Embryos do not survive the biopsy, although this is a rare event
- All the embryos were affected by the genetic condition.
Therefore, a pregnancy cannot be guaranteed using PGD.