Preimplantation Genetic Diagnosis, or PGD, is a specialized laboratory test that is performed during In Vitro Fertilization (IVF) to evaluate one cell of an embryo, called a blastomere, for specific genetic diseases or chromosome abnormalities.  Each embryo created in an IVF cycle is tested separately. The tests will give the doctors information on whether or not the specified genetic diseases or chromosome abnormalities are not, or may not, be present. Physicians can then use the results of these tests to help select embryos for transfer to the mother's uterus.

Preimplantation Genetic Screening (PGS) for aneuploidy is a specific type of PGD used with IVF to identify embryos with an abnormal number of chromosomes, a condition called ‘aneuploidy’. One blastomere of each embryo is evaluated for aneuploidy. This testing allows IVF physicians to transfer to the mother’s uterus those embryos with no detectable chromosome abnormalities. This may increase the chance that a couple will become pregnant during a particular cycle. It may also decrease the chance for miscarriage during the pregnancy and reduce the chance for the couple to have a liveborn baby with a chromosome abnormality such as Down syndrome.

The clinical benefits of PGS are an active area of research. Aneuploidy can occur in any embryo from any couple just by chance, although some couples are at a higher risk. A number of studies using FISH, which until recently had been the industry standard, have demonstrated that PGS may be particularly beneficial for couples meeting any of the criteria below. Traditional FISH testing has known limitations; thus, given the recent advances in PGS technology, including the ability to deliver results on all 24 chromosomes with high levels of accuracy, an even greater benefit from PGS is expected with these newer testing methods. Due to the fact that these technologies are new, clinical data is still pending.

• Women aged 35 and older



• Couples who have had a previous pregnancy with a confirmed chromosome abnormality



• Couples who have had multiple early miscarriages
Studies of couples who have had more than one miscarriage have shown a higher number of embryos with chromosome abnormalities than expected. Some studies have shown an increased rate of pregnancy, a decreased rate of miscarriage, and an increased rate of livebirth following the use of PGS in couples with recurrent pregnancy loss. Others have shown a clear benefit with PGS only in couples with recurrent miscarriage where the mother is 35 years of age or more.

Before considering PGS, couples with a history of 3 or more miscarriages are encouraged to have their own chromosomes evaluated for the presence of inherited chromosome rearrangements, which can be done with a blood test. In approximately 5% of couples with recurrent pregnancy loss one member of the couple is found to have a balanced chromosome rearrangement. A balanced chromosome rearrangement does not affect a person's health, but it does increase greatly the risk for having chromosomally abnormal embryos, the majority of which will be lost in early miscarriage. If a chromosome rearrangement is found, there are specialized Preimplantation Genetic Diagnosis (PGD) methods that can be used to analyze embryos for the unbalanced rearrangement.


• Couples who have had previously unsuccessful IVF cycles
There is data to suggest that aneuploidy rates are increased in the embryos of couples who have experienced repeated IVF cycle failure. Some studies show an increased rate of pregnancy in these couples when PGS is used, although other studies show no clear indication that these couples benefit from PGS. In some of these couples, there may be reasons other than aneuploid embryos for unsuccessful IVF cycles.

PGS is only performed as part of IVF. On the third day after fertilization (Day 3), when the embryos have reached about six to ten cells in size, one blastomere of each embryo is removed for testing in a process called ‘embryo biopsy’.  With current technologies, the blastomeres are prepared using a difficult procedure called 'fixation' and then packaged separately in custom tubes and shipped by medical courier to the testing laboratory. With Parental Support™, we have eliminated the fixation step, making the preparation and packaging of the blastomeres simpler and easier. At the laboratory, each blastomere is individually tested.  Results are reported back to the doctor at the IVF center in time for the embryos to be transferred to the mother’s uterus on Day 5.

On Day 3 when embryo biopsy is performed, all of the cells of the embryo are similar and are not yet separated into different cell types.  Removing one cell for testing does not appear to disrupt or interfere with subsequent fetal development and has not been associated with an increased risk of birth defects. Babies born after PGS and other types of PGD that include embryo biopsy have had a similar rate of birth defects as compared to all other babies in the general population.

Embryo biopsy in the absence of PGS has been shown in some studies to be associated with a reduced rate of implantation. However most doctors feel that if the biopsy procedure is correctly performed, aneuploidy screening more than compensates for this reduced rate of implantation resulting in equivalent or better implantation rates.

No other known adverse effects associated with embryo biopsy or PGS have been reported to date. However, as embryo biopsy is still a relatively new procedure the potential for unknown consequences to a liveborn baby cannot be entirely excluded. 

In most centers FISH is the still the standard technology used for PGS. FISH works by using segments of DNA, called probes, that hybridize, or attach, to a specific chromosome. The DNA probes have been treated with a fluorescent dye so that they light up under a microscope with a different color for each chromosome tested. FISH is used to test a single cell by applying the probes and then using a microscope to count the colored dots: if two dots of a specific color are seen then that chromosome has two copies; if three colored dots are seen then that chromosome has three copies indicating that the cell is aneuploid and abnormal.

The key limitations with FISH are:

1. Only a few probes can be used at a time

 

Only a maximum of five probes can be applied to one cell at one time, which means that only five chromosomes can be evaluated at once. Though the cell can be washed and a second set of probes applied to test additional chromosomes, the ability for the probes to successfully hybridize weakens. Consequently, it is only possible to test a subset of the chromosomes in a blastomere, and if a probe doesn’t successfully hybridize it may result in a misdiagnosis.

 

2. The process of applying and visualizing the probes is prone to error

 

Ultimately, determining whether the blastomere is normal or aneuploid is done by a person looking through a microscope. If the chromosomes are arranged in such a way that the dots overlap, or are difficult to see, it may not be possible to get a good read and a mis-diagnosis may result.

 

Due to these and other limitations, the accuracy of FISH for single cell diagnosis is only around 90%.

Comparative Genomic Hybridization, or CGH, is another new technology capable of evaluating all 24 chromosomes in a single cell. However, it cannot produce results in time for Day 5 embryo transfer.

CGH is usually done by labeling the DNA from a blastomere and DNA from a normal control cell with different colored fluorescent dyes. The samples are mixed and added to a slide with either a set of normal chromosomes or a set of defined DNA probes matching each chromosome. The blastomere and control DNA will hybridize, or attach, to the sections of the slide that are an exact match. If there is an imbalance in chromosome number between the blastomere and the normal control the results will show a different color. For example, when the chromosomes of the blastomere and control cell are balanced, all the probes/chromosomes will be the same color; when there is a missing chromosome in the blastomere, the control DNA color predominates for those probes/chromosomes; when there is an extra chromosome in the blastomere, the blastomere color predominates.

There are a number of limitations with CGH that make it less than ideal for blastomere analysis. First, the processing time for CGH is much longer than for FISH or Parental Support, precluding transfer of the embryos during the same IVF cycle. CGH can take several weeks to produce results, which means that the embryos must be frozen until results are available, a process which may be associated with a reduction in embryo survival.

Secondly, CGH can only detect unbalanced chromosome changes; it cannot detect a complete additional or missing set of chromosomes, such as haploidy and polyploidy which occur in up to 10% of embryos.

Parental Support™ offers two key advantages over conventional PGS:

1. Screening of all 24 chromosomes in 24 hours
2. 99% test accuracy

Historically, the technology used for PGS, called fluorescent in-situ hybridization (FISH), only screens nine chromosomes, leaving the rest of the chromosomes untested. Additionally the accuracy of FISH is considered to be approximately 90% for the chromosomes tested. This means that one in every ten blastomeres may be misdiagnosed due to test error, resulting in either the disposal of a healthy embryo, or the transfer of an abnormal embryo.

Parental Support provides a more accurate test covering more chromosomes than conventional FISH testing methods. More effective testing may improve the chance for IVF success and the birth of a healthy baby.

Prenatal diagnosis is recommended after PGS for two reasons:

1. Although PGS has been performed for years, the technology is still considered investigational
2. Risk of chromosome mosaicism

For these reasons prenatal diagnosis through chorionic villus sampling (CVS) or amniocentesis is recommended to confirm the results

• Chorionic Villus Sampling (CVS)
  Chorionic villus sampling (CVS) is typically performed in the first trimester of pregnancy, between 10 and 12 weeks gestation, by a specially trained obstetrician or perinatologist who removes a small amount of tissue, called chorionic villi, from the placenta either via a catheter placed through the cervix or a thin needle placed through the abdomen.  The cells in this tissue come from the same cells that created the fetus and, thus, should have the same chromosome make-up.  The cells are tested for chromosome abnormalities.

• Amniocentesis
  Amniocentesis is usually done in the second trimester of pregnancy, between 15 and 20 weeks of pregnancy, by a specially trained obstetrician or perinatologist who removes a small amount of the fluid in the amniotic sac via a thin needle inserted into the mother’s uterus.  Fetal cells that float freely within the amniotic fluid are cultured and tested for chromosome abnormalities.