How is PGD performed?
PGD requires patients to undergo an in vitro fertilization (IVF) cycle in order to produce pre-embryos available for testing.
Before the transfer of a pre-embryo back to a woman’s uterus, some cells (called blastomeres) are removed from the available pre-embryos. These blastomeres are evaluated for abnormalities. Following completion of the evaluation (typically one to two days), only the normal pre-embryos are transferred back to the woman’s uterus to establish a pregnancy.
What can be determined about a pre-embryo by PGD testing?
PGD can be used for determining chromosomal health (with PGD-FISH or PGD-CGH) or the presence or absence of specific genes related to hereditary disease (with PGD-PCR).
PGD-FISH (fluorescent in situ hybridization)
The most common chromosomal abnormalities occur when there is an incorrect number of chromosomes, a condition known as aneuploidy. Some chromosomes are more likely than others to become unpaired, which can lead to early embryonic demise (chromosomes 13, 16, 18, and 22) or affect term births, such as Down syndrome (trisomy 21). Numerical mistakes in the sex chromosomes can lead to Turner’s syndrome (XO), or Klinefelter’s syndrome (XXY). Sometimes, chromosomes can break apart and attach to other chromosomes, a condition known as translocation. Translocations have been associated with miscarriage as well as many serious diseases, including some forms of leukemia. Checking for normal chromosome pairing is done using a technique called fluorescent in situ hybridization (FISH), an established component of the preimplantation genetic diagnosis (PGD) testing at our clinics.
PGD-CGH (comparative genomic hybridization)
PGD-CGH is a recent advance in the screening of pre-embryos for chromosomal normality. Its primary advantage is that CGH looks at all 23 pairs of chromosomes instead of a maximum of 12 pairs using PGD-FISH. PGD-CGH may require the cryopreservation of embryos while their cells are being tested; normal embryos are identified, thawed and transferred to the patient typically at her next ovulation cycle.
PGD-PCR (polymerase chain reaction)
The method of analysis for gene disorders is called polymerase chain reaction (PCR). PCR involves a technique that greatly amplifies the genetic code contained in a single blastomere, so that the specific gene or genes being looked for can be detected. Preimplantation genetic diagnosis is significantly enhanced by the use of PCR. Examples of hereditary conditions caused by gene disorders that PCR can help detect include:
- Cystic fibrosis
- Tay-Sachs disease
- Duchenne muscular dystrophy
- Sickle-cell anemia
- X-chromosome-linked disorders
Can PGD of pre-embryos improve the chance of conceiving through IVF?
Until the development of PGD, assisted reproductive technology labs could rely only on the appearance of pre-embryos to decide which ones were suitable for implantation into the patient’s uterus. The advent of PGD proved that normal-appearing pre-embryos frequently have an abnormal number of chromosomes, rendering them incompatible with the development of a normal pregnancy.
In summary:
- PGD allows embryologists to select pre-embryos that are balanced for the tested chromosomes.
- PGD testing enables the implantation of only selected pre-embryos, increasing the chance of pregnancy.
- In addition, because the quality of the pre-embryo has been proven, fewer pre-embryos are needed to establish a successful pregnancy, greatly diminishing the chances of a multiple pregnancy.
Indeed, the use of PGD by the fertility specialists at the Southern California Reproductive Center has been instrumental in bringing many babies into the world.
Are there risks in using PGD?
While manipulating and handling embryos will always have some measure of risk, it is important to note that our center has the most extensive experience of any in the USA. The real risk is in NOT using PGD, and then having a miscarriage or carrying a child to term with a serious genetic disorder that could have been detected through PGD testing.
What does PGD cost?
The cost of PGD depends on the specific types of tests desired or requested (see above for the variety of genetic tests that comprise the PGD process). At SCRC, we are able to offer you several IVF financing plans to make the cost manageable and affordable. Please talk to one of our staff members for details.
What does the future hold for genetic testing of embryos?
In 1999, the genetic code of an entire human chromosome was sequenced (mapped). This revolutionary development in medical science allowed the continuing discovery of the sequence of the code that comprises specific units of genetic information. Genes are responsible for directing the construction of every cell in our body, down to the tiniest molecular detail. Therefore, it is reasonable to expect that the function of all genes, and the detection of abnormal genes, eventually will be categorized. This knowledge may in turn be used to reduce or eliminate susceptibility to hereditary disorders, infectious diseases, and cancer. Someday, only a single pre-embryo will be needed to establish a pregnancy — and it will be a healthy pregnancy.
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