In Vitro Fertilisation: Understanding Preimplantation Genetic Diagnosis (PGD)

Preimplantation Genetic Diagnosis (PGD) is a process sometimes used in conjunction with in vitro fertilization (IVF) that allows couples to screen their embryos for genetic diseases and disorders before implanting the embryo back into the mother’s uterus.

PGD may be used for a number of reasons:

  • To check for inherited genetic diseases
  • To check for chromosomally abnormal embryos
  • To select which embryos are the best candidates for implantation back into the uterus
  • PGD is a controversial issue that has numerous ethical issues surrounding it. While some believe it is a beneficial technological advancement to ensure a healthy baby, others believe it is less beneficial than it is touted to be in certain cases, and yet others believe it is a recipe for ethical disaster in the wrong hands.

    How is PGD Done?
    PGD is preformed during IVF after a woman’s eggs have been harvested and fertilized but before they are implanted into her uterus. When the fertilized egg has reached somewhere between the 4- and 10-cell stage of development (usually the 8-cell stage, around day 3), a hole is made in the shell of the embryo and a pipette is used to gently remove one cell on which testing will be done. This technique is called embryo or blastomere biopsy.

    The DNA is removed from the cell and tested for the genetic disease or disorder in question. It may also be checked for overall chromosomal normality. Any diseased or abnormal embryos are destroyed.

    Testing is usually completed in one day and the next day the healthy embryos are ready for implantation back into the mother’s uterus. Sometimes the embryos will be monitored for several extra days for health and normalcy and implanted in the mother as a blastocyst (5-7 day old embryo). This is called IVF with blastocyst culture and transfer.

    What Can PGD Test For?
    Diseases and disorders that PGD can check for may vary by clinic, depending on what tests they offer. That being said, on the request of the couple undergoing IVF, it is possible to test for numerous conditions classified as either: chromosomal disorders, single gene defects or sex-linked disorders. If you have a specific disorder or disease that you would like to test for based on your own family history or concern, it is best to check with your doctor or clinic to see if the test is possible or offered. A sample of conditions that can be tested for are:

  • Breast And Ovarian Cancers
  • Chromosome Translocation
  • Congenital Adrenal Hyperplasia (CAH)
  • Cystic Fibrosis
  • Down's Syndrome
  • Early Onset Alzheimer Disease
  • Fragile X Syndrome
  • Hemophilia
  • Huntington’s Disease
  • Klinefelter Syndrome
  • Lou Gehrig's Disease
  • Muscular Dystrophy
  • Niemann-Pick Disease
  • Polycystic Kidney Disease
  • Sickle-Cell Anaemia
  • Tay-Sach's Disease
  • Turner's Syndrome
  • Wilson Disease
  • Who Can Use PGD?
    When PGD was first developed, it was only meant to be used by couples who had embryos with a high risk of developing genetic disorders. However, as technology becomes more advanced and the practice is researched more, PGD is becoming more common in some countries, especially in the United States, while being kept under tight restriction in others.

    Even within clinics, assess may be restricted in different ways. General guidelines for who should be offered PGD in most clinics are:

  • Couples with family histories of inherited genetic diseases
  • Couples where either partner is a carrier of chromosomal translocations (their chromosomes in their genes are “mixed up” – part of one chromosome has broken off and joined with another)
  • Women who have experienced recurrent miscarriages
  • Women over the age of 38 (which is considered an advanced female age for bearing children) attempting IVF
  • Any couple who has experienced repeated failed attempts at IVF (usually 3 or more)
  • Any couple where a man’s sperm count is low that it is necessary to perform ICSI
  • Does PGD Make a Difference? Is PGD Effective?
    Since PGD may be done for different reasons, the effectiveness of the treatment depends largely on the reason for which it’s being used.

    Sometimes PGD is used by a couple where at least one partner knows he or she is carrying a gene that could put their child at risk for a genetic disease or disorder. In this case, PGD is quite effective since it allows testing for this specific gene in the embryos, and any embryo carrying the gene may be destroyed. This ensures that any child conceived by this couple (using IVF and PGD) will not be at risk for the disease or disorder.

    Another case in which PGD is very effective is when a couple who is trying to conceive knows that at least one partner is a carrier of a chromosomal translocation. This means that the chromosomes in this partner’s genes are basically “mixed up” – a piece of one chromosome is broken off and attached to another. The parent may be normal, but a chromosomal translocation may cause complications when he or she tries to start a family by causing abnormal embryos, miscarriages, and birth abnormalities. PGD is effective here because all embryos will be tested for chromosomal abnormalities, and only normal embryos will be transferred back into the uterus, ensuring a healthy baby without any birth defects.

    Lastly, PGD may also be used for aneuploidy screening. This means that all embryos are checked prior to reinsertion into the uterus simply because the woman is at an advanced female age for bearing children (generally thought to be over 38) or the couple has already had multiple failed attempts at IVF. The logic behind this is that a small percentage of human eggs are naturally abnormal. This percentage increases with increasing female age. An older female therefore has a higher chance of producing an abnormal embryo than her younger counterpart. The probability of an abnormal embryo can reach up to 50% in a woman at or approaching 40. So, by checking the embryos before transfer using PGD, all abnormal embryos can be discarded, and only normal ones will be transferred into the woman’s uterus. This means that less embryos are transferred (meaning less risk of multiple birth), and there may even be higher pregnancy rates because of the quality of embryos used. However, there has not been consistent proof that PGD actually results in more live births than IVF without PGD when PGD is used for aneuploidy screening. Miscarriage rates are lower in some, but not all studies, and sometimes PGD can give false test results.

    This means that it is not necessarily better to use PGD just because you have had previous IVF cycles that did not work or because you are a woman over 38. PGD for this reason has not been proven to increase chances of pregnancy. However, if you know you have a chromosomal translocation or you are carrying a gene for a genetic disease or disorder, and your potential child is at a high risk of inheriting medical problems, PGD is useful for increasing the chances of a live birth and a healthy baby.

    Ethical Issues Associated with PGD

    PGD is very controversial because of the many ethical issues involved with this procedure. Many feel that it is unethical to create an embryo outside of the body and then destroy it. This is especially so because PGD is not 100% accurate, which could lead to the destruction of a normal embryo. Also, PGD does not guarantee that a child will be born without a disease or disorder since genes are not the only things that cause medical problems – fetal environment has a large impact as well.

    However, others praise PGD because it can detect potential problems before a pregnancy has even begun. The current standard procedure used to check for genetic disorders during pregnancy is amniocentesis, and this is done much later – during the second trimester. This carries a greater risk to the fetus, and if the parents decide to abort the pregnancy based on amniocentesis results, this is a much more emotionally painful decision.

    There is also worry that PGD use may place a greater emphasis on “healthy” versus “unhealthy” people in our society. Yet advocates point out that PGD may allow incidences of specific diseases to be significantly lowered or eliminated all together.

    Since PGD can be used to select the sex of a baby, it is also a worry that this will allow parents to favour one sex over the other. In societies where there is already a social preference towards one sex (usually males), allowing parents to choose the sex of their baby may lead to an imbalance of the social makeup of that society. On the other hand, PGD allows for avoidance of sex-linked disorders and allows parents to choose how their family will be made up.

    Finally, PGD offers hope to families where one member is sick and in need of a stem cell transplant. PGD would allow parents to identify an embryo that would be a perfect match for the ailing member. Once the child is born, the stem cells from the umbilical cord would be collected and used for transplant at no harm to the mother or child. This is comparison to harvesting bone marrow, which is a highly invasive and painful procedure. However, there are worries that using PGD children would be created for their “spare parts.”

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