Preimplantation Genetic Screening (PGS) and Preimplantation Genetic Diagnosis (PGD)

In vitro fertilization (IVF) has evolved dramatically over the last three decades, with more than six million babies now born through assisted reproductive technology. The most recent focus is no longer simply on assisting individuals to achieve pregnancy, but, advancements in IVF and genomics have shifted the goal to be achievement of a single healthy baby. For years, after retrieval and fertilization of a woman’s eggs, multiple embryos were transferred to the uterus in hope of achieving one or more implantations. Unfortunately, one of the consequences could be multiple gestations which could lead to premature delivery.

Current technology has now provided an improved methodology. Following fertilization, embryos are now biopsied and frozen or “vitrified.” If the biopsy reveals that the embryo is chromosomally normal, it is thawed and transferred to the uterus several weeks later. Embryo biopsy with Preimplantation Screening (PGS) can be utilized to test for Down syndrome and other chromosomal issues. In less than two years, this technology has revolutionized reproductive medicine. Miscarriages are nearly eliminated and patients are being transferred one single screened embryo.

Preimplantation Genetic Diagnosis (PGD) involves the same process of fertilization, biopsy, and vitrification, but rather than screen for chromosomal abnormalities, DNA analysis is performed to determine whether the embryo carries a mutation for a specific disease for which the couple is at risk. For example, if both parents carry a mutation for Cystic Fibrosis, the embryo could be tested, and the healthiest embryo could be transferred.

At RMA of New York, we are proud to offer to our patients preimplantation genetic screening (PGS) and preimplantation genetic diagnosis (PGD). Below you’ll find the questions that we most commonly answer for our patients who are wondering whether preimplantation genetic screening and diagnosis is right for them.

 

 

Who benefits from preimplantation genetic diagnosis (PGD)?

There are a number of situations that lead us to recommend PGD to our patients, including when one or both partners has a history of heritable genetic disorders, and when one or both partners tests positive for a mutation that could cause disease in the offspring. Examples could include predisposition to recessive genetic diseases, such as Tay Sachs or cystic fibrosis, X-linked genetic diseases, such as hemophilia or Duchenne Muscular Dystrophy, and autosomal dominant genetic diseases, such as Marfan syndrome, Huntington’s disease, as well as cancer predisposition syndromes, such as Lynch syndrome and BRCA. Couples with predispositions to these genetic disorders may undergo PGD to prevent having a child with the disorder.

What type of conditions can be screened for using preimplantation genetic screening (PGS)?

There are a number of chromosomal abnormalities that can be identified during the PGS process. These include alterations in the number of chromosomes, leading to conditions such as Down syndrome, Turner’s syndrome, and unbalanced chromosome complements that result from structural chromosome rearrangements in the parents’ genes that can cause birth defects, mental retardation and/or miscarriage. A key benefit of using PGS is screening for aneuploidy, or chromosomally abnormal embryos, which are identified and then not transferred back during an IVF cycle. This is important because aneuploidy is a significant contributing factor in implantation failure and spontaneous miscarriages, and is likely responsible for the majority of IVF failures. Aneuploidy rates increase with increasing maternal age which is why older women have lower pregnancy rates and higher miscarriage rates in an IVF cycle.

I’m a woman over 35. Should I have preimplantation genetic screening (PGS) done?

Women over the age of 35 are at increased risk of having chromosomally abnormal embryos. Guidelines have been established by the American Society for Reproductive Medicine (ASRM) for the number of unscreened embryos to transfer in IVF cycle. In order to maximize success rates of an IVF cycle, these guidelines recommend the transfer of 2 unscreened day 5 embryos in women over the age of 35. Consequently, this places the couple at risk for a twin pregnancy, and the inherent risks and consequences associated with a multiple gestation pregnancy. Recent studies demonstrate that transfer of a single screened euploid embryo, which is a chromosomally normal embryo, has an equivalent pregnancy rate as transfer of two unscreened embryos. As a result, due to the improved implantation and pregnancy rates, couples utilizing PGS generally undergo a single embryo transfer, which therefore almost eliminates the risk of a twin pregnancy. Couples undergoing IVF over the age of 35 have been increasingly utilizing PGS with a single embryo transfer to maximize their chance of a healthy singleton full-term pregnancy.

I’ve had several miscarriages. How does preimplantation genetic screening (PGS) help me?

It is estimated that approximately half of miscarriages that take place during the first trimester of a pregnancy are a result of a chromosomal abnormality. These abnormalities may be a result of advanced maternal age, as chromosomal aneuploidy rates increase with increasing maternal age, and is why older women have higher miscarriage rates. PGS can identify embryos that are most likely to possess the normal chromosomal complement and select them for embryo transfer, thus greatly reducing the risk of miscarriage. The ability to prevent an abnormal pregnancy allows patients to avoid the physical and emotional suffering of a miscarriage and ultimately is a more patient-friendly approach.

Can I have preimplantation genetic screening (PGS) done to identify an embryo’s gender before implantation?

Gender determination can be of importance in families that are attempting to balance their families. PGS can be used to help couples select for the desired gender.

When is preimplantation genetic testing performed?

Following egg retrieval and fertilization, the embryos are grown to the blastocyst stage. At this point, they possess greater than one hundred cells. The embryologist uses a laser to carefully dissect several cells that are being shed by the embryo, and these cells are then sent to the laboratory to determine whether the embryo has a specific genetic disorder and/or whether the embryo is chromosomally normal. These embryos are generally frozen until the genetic tests are available, and then the healthiest embryo can be thawed and transferred the following month. Commonly, a single healthy embryo is chosen for transfer.

Preimplantation genetic diagnosis and screening uses genetic technology to minimize the risk of nonviable pregnancies resulting in miscarriage, as well as the emotional and ethical challenges presented by prenatal diagnosis or of having a child born with a heritable disease. The use of this technology is just one way that RMA of New York is using scientific breakthroughs to help patients achieve their family-building goals in the most supportive and effective way possible.

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