PREIMPLANTATION GENETIC DIAGNOSIS (PGD/PGS)
Preimplantation Genetic Diagnosis (PGD) may be used in cases of fertility issues such as failed implantation, poor embryo development or frequent miscarriages. However, non-fertility but equally important conditions exist where PGD plays an important role in assisting patients. For example in patients with known adverse genetic conditions at risk of passing them to the offspring, or just for the sake of “balancing” a family where a specific gender is absent. PGD can be used to assess the quantity and normality of chromosomes when it is suspected that they may be involved in the problem.
Also known as Preimplantation Genetic Screening (PGS), the purpose of this process is exclusively to evaluate the chromosomal makeup of an egg/embryo. Because the majority of miscarriages are due to chromosomal abnormalities, many couples undergo Preimplantation Genetic Diagnosis in order to select embryos that are chromosomally “normal” and increase their chances of IVF success. An egg/embryo with too many or too few chromosomes will either fail to implant, lead to a miscarriage at different stages of pregnancy or it may result in a chromosomal birth defect such as Down Syndrome or Turner Syndrome.
GENETIC TESTING (PGD/PGS)
WHO ARE CANDIDATES FOR PGD/PGS?
MISCARRIAGE AND PREGNANCY LOSS.
Patients with unexplained infertility, who have repeatedly suffered from miscarriages or early pregnancy losses, may have a high incidence of chromosomal or genetic conditions. PGD technology may be able to identify normal embryos which are selected for transfer and display a higher probability of implantation.
Age and medical history are important factors to consider using PGD/PGS. As patients get older their egg quality deteriorates making them more susceptible for chromosomal abnormalities. The incidence of chromosomal derived conditions (Down Syndrome, Turner Syndrome, Klinefelter Syndrome, Patau Syndrome to name a few) are all derived of an abnormal number of chromosomes where advanced age plays a definite role. For example the incidence of Down syndrome in women over 40 is about 1 in 60 while in women under 25 is about 1 in 2500. Other chromosomal conditions (deletions, inversions, etc) which cause fertility problems (failed implantation, miscarriages) may be present but are not age related.
Patients with medical history of mutations or other genetic conditions such as Cystic Fibrosis, Thalassemia, Huntington Disease, Sickle Cell Anemia, etc. are also candidates for PGD. The unaffected embryos can be identified and used for transfer.
INCREASING PREGNANCY RATES.
PGS can improve pregnancy rates by identifying those embryos that are most likely to make a baby. The additional cost may negate the advantages for some couples.
Family balancing or gender selection is another reason for having genetic testing done. In fact, PGD/PGS is really the only accurate method of gender diagnosis available. Because the gender chromosomes are identified during PGD, it is possible to select a “male” or “female” embryo for transfer to the uterus.
DIFFERENCES BETWEEN PGD AND PGS
PGD is the general term for preimplantation genetic diagnosis. It is applied to identify the presence of a specific condition (a genetic disease) or extra, absent or abnormal
chromosomes. In case of chromosomes, PGD screens 2, 5, 8, 12 or all pairs of chromosomes. After the egg is fertilized and progresses to the embryo stage, on day 3 or 5 of age one or more cells are removed from each embryo to be tested. The cells are then analyzed to find the genetic mutation sought after or the extra, missing or abnormal chromosomes. Embryos with an extra or missing chromosome are referred to as chromosomally “Aneuploidy”. Embryos with the normal number of chromosomes are referred to as “Euploid”
As stated earlier PGD is designed to identify a specific gene or a determined number of chromosomes. The higher the number the higher the chance of an errors so it is far from 100% accurate.
In the last few years a modification of PGD, known as Preimplantation Genetic Screening (PGS), has emerged specifically detect aneuploidies (abnormal chromosome numbers) and it analyzes all 23 pairs of chromosomes for a far more comprehensive and accurate analysis. So it indicates if all chromosomes are present and also tells us the gender of each embryo.
As nearly all implantation failures and miscarriages are originated form chromosomal abnormalities it is possible with PGS to detect normal embryos for transfer and increase the probability of implantation. Even pregnancy rates for older women are improved as only good quality embryos are transferred.