PGD: The Best Choice for Some Parents

Getting pregnant and having a baby boils down to what each parent brings to the conception via their gametes -- oocytes (eggs) from the woman and sperm from the man. Each of these cells contains the characteristics that a new, unique human being will inherit and develop. While advanced reproductive technology can do a lot to work around many counterproductive circumstances, a parent's genetics cannot be changed. However, preimplantation genetic diagnosis (PGD), performed in conjunction with in vitro fertilization (IVF), can provide answers to hopeful parents-to-be in several different circumstances.

PGD is not simply one test, but a combination of techniques administered by highly skilled embryologists, scientists who specialize in embryo development. In brief, PGD allows an embryologist to view the genetic makeup of a single cell from an embryo created with IVF. The information gained from analyzing the embryo's cell is used to determine which embryos will be transferred to the woman's uterus.

Because PGD is not an automated test (as is the case for many fertility hormone blood tests, for example), and because the technology is new and very skill-dependent, the PGD process is expensive. However high the current cost, for many people wanting to become biological parents, PGD may provide their final option for conceiving a healthy baby.

Who Can Benefit from PGD?

  • parents who know from having previous offspring that their genes carry certain disorders, for example, cystic fibrosis, sickle cell anemia, some types of muscular dystrophy
  • parents who have a known family history of certain disorders
  • parents who have experienced recurrent pregnancy loss
  • some parents who have been diagnosed with unexplained infertility

How is PGD Performed?

PGD inherently requires that the embryos first be created through IVF in a lab. In order for an embryo to be examined with the fluorescence in situ hybridization (FISH) or DNA analysis, a single cell (and sometimes, a second cell) must be removed from the embryo on approximately Day 3 of its development. At the time of this cellular biopsy, most embryos have between four and 12 distinct cells. There is some risk at this point, estimated to be below one percent, of accidentally damaging the embryo in the biopsy process. For people who worry about carrying some genetic conditions and depending on the total number of embryos produced in the initial IVF process, this risk could be well below the potential loss they face without PGD.

As renowned embryologist Michael Tucker, PhD, puts it, "As with many things in life, patients and physicians must do a cost-benefit assessment based on the risk to the embryo weighed against the medically-driven reasons for performing PGD." Tucker, who is the Scientific Director of Shady Grove Fertility's PGD Center, is one of a few embryo experts who have been fine-tuning PGD technology for the past decade.

Once the embryo is biopsied, the embryologist examines the cell using the latest available tests. Scientists like Michael Tucker and Dr. William G. Kearns, Program Director of Shady Grove Fertility's PGD Center, are continually advancing the types of conditions that can be analyzed via PGD. At this time, PGD testing can detect aneuploidy (too many or too few chromosomes) and structural abnormalities of the cell's chromosomes, as well as many of what are sometimes called single gene disorders.

Is PGD Foolproof?

No technology is foolproof. Some predictive limitations of PGD can be made up for by peforming either chorionic villi sampling (CVS) or amniocentesis later in a pregnancy. Adding these two well-established tests in either the first or second trimester of pregnancy can confirm or dispute the PGD results. False readings can occur with PGD due to:

  • lab or human error
  • mosaicism

    • Mosaicism is a situation in which an embryo may have both genetically normal and abnormal cells. If the cell that is retrieved during biopsy happens to be normal, there is no way to know if the other cells in the embryo are also normal.

    Aside from inaccuracies that may arise during the testing process or from mosaicism, some of PGD's limitation is related to the fact that it's such a new science.

    "The maximum number of chromosomes we can routinely examine for aneuploidy right now are up to 11 of 24," Michael Tucker explains. Part of the problem is timing. "We go in on the third day of development," Tucker continues, "pull out one of those cells, and then we have until Day 5 -- about 48 hours -- to come up with test results about the condition of that embryo." At that point, the embryo must be transferred to the woman's uterus if it is to continue a relatively normal course of development.

    Additionally, it simply is not yet possible to detect every single gene disease that exists.

    Tucker sums it up, "As more is learned about the genetic location of inheritable conditions, and testing technology is improved and made swifter, PGD will be able to screen for a wider array of diseases; therefore, its application will be more readily justified."

    Best for Now

    PGD is not available everywhere. However, some labs such as the Shady Grove PGD Center provide analysis services to other fertility clinics, in addition to serving Shady Grove's own fertility patients. Biopsied cells can be transported via special overnight delivery virtually anywhere.

    At this time, PGD is not the answer for everyone, but it does provide the greatest chance for avoiding the transmission of certain genetic diseases to offspring and can help many patients solve the riddle of recurrent miscarriage and unexplained infertility.