The following gives you information about preimplantation genetic diagnosis (PGD). This page tells you what PGD is, how and why it is done, what happens throughout the process, and possible advantages and risks of the treatment. This information is designed to be used alongside the discussions you have with your health care professionals and help you to ask the questions that are important to you.
Click here for a printable leaflet containing the information on this page.
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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. PGD is used when one or both parents has a known genetic abnormality and diagnostic testing is performed on an embryo to determine if it has also inherited the abnormality.
PGD involves the use of assisted reproductive technology (ART). Eggs are obtained and fertilised through in vitro fertilisation (IVF). Once fertilised, the embryos develop for 2-3 days and then cells are removed from each embryo. The genetic material (DNA or chromosomes) within one cell of the embryo is then tested for the genetic or chromosomal abnormality. Up to two unaffected embryos are then transferred to the woman’s uterus to implant. If successful, the procedure will result in pregnancy and the child should not be affected by the condition for which it was tested.
Couples who request PGD are at risk of passing on a serious genetic condition to their children. Parents may have these conditions themselves or be carriers of the conditions. They may already have a child affected with the genetic condition or they may have even experienced the loss of a child or pregnancy affected with the condition.
Primary candidates for PGD include:
- Carriers of a serious autosomal recessive condition: For carriers of an autosomal recessive condition, there is a 1 in 4 (25%) risk of each pregnancy delivering an affected child.
- Carriers of a serious autosomal dominant condition: For carriers of an autosomal dominant condition, there is a 1 in 2 (50%) risk of each pregnancy delivering an affected child.
- Couples with chromosome disorders.
- Couples with a family history of serious sex-linked disorders: Couples with a family history of a sex-linked disorder have a 1 in 4 (25%) risk of each pregnancy delivering an affected child (half of all male children).
Therefore, PGD may be recommended if:
- you or your partner has a family history of a serious genetic condition and/or
- you and your partner already have a child with a serious genetic condition and/or
- you and your partner have had a number of pregnancy terminations because your baby had a serious genetic condition.
PGD is available for three broad categories of conditions including:
- Single gene defects for which PGD testing is available.
- Chromosomal abnormalities, including inversions, deletions, aneuploidy (an abnormal number of chromosomes), and translocations (where a piece of one chromosome either becomes attached to another chromosome or swaps places with a segment from another chromosome).
- Sex-linked conditions for which it is not yet possible to test for the specific genetic mutation.
For many genetic conditions, cures do not currently exist and are not likely to be found soon. Additionally, available treatments often carry substantial risks or have adverse effects. Even if effective, affordable treatment were guaranteed to be available, the psychological impact of late onset conditions on those at risk and on their families is significant. Therefore, the option of preventing the condition may be preferable to waiting for a possible cure to eventually become available.
PGD also provides:
- An alternative to prenatal diagnostic testing: Couples avoid the distress associated with diagnosis, particularly if late in an established pregnancy, as well as the risk of miscarriage associated with such testing.
- An alternative to termination of pregnancy: If prenatal testing (through amniocentesis or chorionic villus tests) reveals a genetic abnormality, the options available to parents are to have a child with a genetic condition or to undergo a pregnancy termination. This is a difficult and often traumatic decision. However, PGD is performed before pregnancy begins, thus eliminating these difficult decisions.
- The opportunity to conceive a pregnancy that is biologically the parents’ own and yet unaffected by a genetic condition in the family: Couples for whom the option of prenatal testing and termination is not an acceptable choice are unlikely to want to take the risk of a child having a genetic condition. In the past, potential parents with a genetic condition or those who know that they are carriers frequently chose adoption, embryo donation, or surrogacy or chose not to have children in order to avoid the risk of passing on the condition. PGD now allows these couples the opportunity to have a child free of the condition.
Couples considering this treatment should talk to their GP to discuss the options available. Before requesting PGD, potential parents should consult their local clinical geneticist or genetic counsellor to evaluate the risk of having a child with the inherited condition. Tests should be performed to determine the genetic defect causing the condition of the affected parent or family member. Couples must also ensure that PGD can identify that defect in an embryo biopsy and confirm that a PGD licence has been issued by the Human Fertilisation and Embryology Authority (HFEA) Licence Committee for that particular condition before proceeding further.
Preparation for PGD consists of several stages. These typically include:
- Referral to a PGD clinic by the couple’s clinical geneticist or genetic counsellor.
- An initial clinical consultation.
- Applying for funding for procedural and drug costs.
- Obtaining the potential parents’ consent for the storage and use of gametes and embryos.
- Laboratory testing and preparatory investigations (including health history, ultrasound scan, blood tests, semen analysis).
- Treatment cycle.
Preparation time will vary, but on average it will take 8-12 months after the initial consultation before a couple will start treatment. Once in a treatment cycle, the process takes about nine weeks.
The cost of PGD is typically split into two parts: procedural costs (consultations, laboratory testing, egg collection, embryo transfer, ultrasound scans, and blood tests) and drug costs (for ovarian stimulation and embryo transfer).
PGD combined with IVF will cost £6,000 - £9,000 per treatment cycle.
PGD clinics can apply to a couple’s local Primary Care Trust (PCT) or Local Health Board (LHB) to request funding for treatment. Each referral is considered individually. A couple’s own Strategic Health Authority (SHA) or Primary Care Trust (PCT) may have separate criteria, which has to be taken into account before they will consider funding for PGD. A particular PCT may refuse funding if a couple already has unaffected children or if the woman is over a certain age.
If funding is not available (if a couple is not eligible for NHS Funding or their PCT declines funding), then a couple can pay for a treatment cycle themselves.
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 one 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 a process 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. This technique is used mainly for chromosome abnormalities or sex-linked conditions.
- Intracytoplasmic Sperm Injection (ICSI): 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 2-3 days until the cells have divided and the embryo consists of around 8 cells. An embryologist removes one or two of the cells (blastomeres) from the embryo.
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.
Instead of removing and testing one or two cells from a 2-3 day old embryo, clinics may allow the embryo to develop for 5-6 days, when there are 100-150 cells. Because more cells can be removed at this stage from the trophectoderm (the cells which will form the placenta), this type of biopsy could possibly lead to a more accurate diagnostic test.
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 in the family.
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. No more than two embryos 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 can be frozen for later use.
Those embryos that are affected by the inherited condition are allowed to perish.
Step 8: Pregnancy Test
About two weeks after the embryo transfer, the woman is given a pregnancy test. A positive pregnancy test means that at least one embryo has implanted.
Click here to view a cartoon leaflet of the procedure.
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.
Given the limitations of current PGD technology, it is recommended that patients undertake prenatal testing (amniocentesis or chorionic villus sampling) if PGD results in a pregnancy.
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.
- Removing the cells for biopsy damaged the embryos.
- All the embryos were affected by the genetic condition.
Therefore, a pregnancy cannot be guaranteed using PGD.
Risks of treatment
Most of the risks involved in PGD treatment are similar to those for conventional IVF:
Fertility drug reaction: mild reactions may involve hot flushes, feeling down or irritable, headaches and restlessness; symptoms usually disappear after a short time.
- Multiple pregnancy: risk of complications is higher in a twin pregnancy.
- Ovarian Hyperstimulation Syndrome (OHSS): Symptoms include abdominal pain and swelling, shortness of breath, nausea, vomiting, and a reduction in urine output; can result in mineral imbalances in the blood and clotting problems.
- Pelvic infection.
- Miscarriage: risk of a miscarriage after IVF is no higher or lower than after a natural conception.
- Ectopic pregnancy: ectopic pregnancy can cause vaginal bleeding, low pregnancy hormone levels, and miscarriage.
Risks to babies born following PGD
No strong evidence currently exists to suggest that babies conceived through this process are at any greater risk of abnormality than babies conceived through other forms of assisted reproduction.
Several studies have concluded that there do not appear to be any major side-effects to the PGD treatment. However, there is evidence that babies conceived through IVF are more likely to be:
- Born prematurely.
- Weigh less than naturally conceived babies born at the same age.
Follow-up data remains limited and long-term outcomes are unknown. It is important to continue to monitor the progress of children born following PGD to gather information about the potential impact of the treatment.
Guy's and St Thomas' Centre for Preimplantation Genetic Diagnosis
Human Fertilisation and Embryology Authority (HFEA)