Preimplantation Genetic Diagnosis
First done in 1988, preimplantation genetic diagnosis (PGD) allows couples undergoing in vitro fertilization (IVF) for infertility to screen their embryos for a host of genetic diseases and disorders. While many see this relatively new technology as an ideal way of ensuring a healthy baby, others view PGD as a recipe for disaster.
How is PGD Done?
Preimplantation genetic diagnosis is used during an IVF cycle to identify specific embryos that carry abnormal genes. PGD can also be done to help select embryos of a particular sex, usually to avoid sex-specific diseases but it can also be done for family balancing. PGD is normally recommended for couples where one or both of the parents are a known carrier of a genetic disease (i.e. hemophilia or cystic fibrosis).
PGD is usually performed after a woman’s eggs have been harvested and fertilized by her partner’s sperm but before the eggs have been transferred back to her uterus. When the fertilized egg has reached between the four and 10-cell development stage, one or two cells (known as blastomeres) are gently removed from each embryo using a microsurgery technique similar to that used in ICSI. The DNA from the removed blastomeres is then analyzed for any genetic diseases or disorders. If any are found, then those embryos are destroyed. Only those embryos with non-diseased genes will be transferred back to the mother.
What Genetic Diseases Can PGD Test For?
Numerous diseases and disorders classified as either chromosomal disorders, single gene defects or sex-linked disorders can be tested for through PGD. Specific chromosomes are tested for specific disorders, including:
- Chromosome 13: Breast and ovarian cancers, deafness, Wilson Disease
- Chromosome 15: Marfan Syndrome, Tay-Sach's Disease
- Chromosome 16: Polycystic kidney disease, Alpha thalassemia
- Chromosome 17: Charcot-Marie-Tooth Disease
- Chromosome 18: Niemann-Pick Disease, pancreatic cancer
- Chromosome 21: Down's Syndrome
- Chromosome X: Duchenne muscular dystrophy, Turner's Syndrome, Fragile X Syndrome
- Chromosome Y: Acute myeloidleukemia
Who is Eligiable for PGD?
When PGD was first offered to the public, it was meant to be used only by those couples whose future children were at "high risk" of developing a genetic disorder. However, PGD was originally only offered at a select few infertility clinics worldwide.
As more and more clinics begin to offer PGD, this technology is increasingly becoming available to anyone who would like to use it, at least in the United States. Many countries still have severe restrictions on this procedure. As a result, couples all over the world criss-cross the globe to be treated at a fertility clinics where PGD is performed at the patients’ request.
Those clinics that do restrict the use of PGD generally follow these guidelines as to who should be offered PGD:
- Women over the age of 35 who have had miscarriages or failed attempts at becoming pregnant through IVF
- Women who have experienced recurrent miscarriages
- Any couple who has repeatedly been unable to become pregnant through multiple IVF cycles
- Any couple where a man’s sperm count is low that it is necessary to perform ICSI
Preimplantation Genetic Diagnosis Ethics
It is not surprising to learn that many people have ethical concerns over both PGD and IVF. One common concern is that PGD will lead to more emphasis on society valuing "healthy" people over those deemed "unhealthy". Yet advocates of PGD point out that this technology may allow incidents of some diseases or disorders to be significantly lowered or even eliminated. Many also take issue with the idea of an embryo being created outside of the body and then being destroyed. Moreover, PGD test results are not 100% accurate. Even if a chromosomal abnormality is found, it does not guarantee that the child will be born with a particular disease or disorder.
On the other hand, many people praise PGD because it can detect potential problems before a pregnancy has even begun. Currently, amniocentesis is the standard procedure used during pregnancy to detect genetic anomalies. However, this procedure is not performed until the second trimester and carries a greater risk to the fetus. Additionally, if a couple decides to abort their child based on the results of the amniocentesis, it is a much more difficult and emotionally involved decision.
PGD can also benefit families where one child is sick and requires a stem cell transplant. Through PGD, a couple would be able to identify an embryo that would be a perfect match for their ailing child. Once the second child was born, stem cells from her umbilical cord would be collected and used for the transplant at no inconvenience to the child or mother, unlike bone marrow stem cells where collection is a painful and invasive procedure. Again, though, there are concerns that some babies would be born just for their "spare parts".
The issue of sex selection and PGD is also a thorny one. While many like the idea of being able to choose how their family will be formed or hope to avoid a sex-linked disorder, others argue that sex selection encourages people to favor one sex over the other. In countries where there is already a social preference towards one sex, often the male, there is worry that allowing couples to choose the sex of their baby would cause an imbalance in the social makeup of that society.
PGD and Pregnancy
One concern of many couples is whether or not the procedure will affect their chances of pregnancy or, more importantly, the health of their embryo. Because the procedure is done so early in the developmental process, at a time when any cell from the embryo can potentially be removed and go one to create another embryo on its own, PGD does not cause any harm to the developing embryo. How PGD affects your chances of a successful pregnancy, though, are debatable.
Some people feel that performing PGD and manipulating the embryo in some way is likely to lessen your chances of a pregnancy through IVF occurring. However, others disagree, believing that pregnancy success rates actually increase through the use of PGD. This is because only those embryos that have been shown to be in good health are transferred back to the mother.
Since genetic abnormalities are the most common reason for a miscarriage, transferring only those embryos without any abnormalities decreases the risk of a miscarriage. However, there are only so many genetic disorders and diseases that PGD can detect at this time. It is still possible to transfer an embryo with genetic problems that just can’t be assessed by PGD.
PGD is not a routine part of IVF. To have PGD done, couples will have to pay an extra fee, on average between $4500US and $7000US, on top of their IVF fees. However, if you or your partner are a carrier of a genetic disease or disorder that PGD can test for, this fee may be worthwhile for the peace of mind PGD can offer you.