An Insight Into Preimplantation Genetic Screening and Preimplantation Genetic Diagnosis

May 2, 2024
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There are a host of novel technologies available in the fertility space. Each of these plays a unique role in augmenting the chances of pregnancy for couples unable to conceive. Today, I will spotlight two of the most popular technologies that have taken centre stage in recent years: preimplantation genetic screening (PGS) and (PGD) preimplantation genetic diagnosis.

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What Are PGS and PGD?

  • Preimplantation Genetic Screening

PGS is a test that examines the chromosomal construct of an embryo. It allows embryologists to determine whether the number of chromosomes present in an embryo are adequate. PGS does not necessarily trace for specific diseases; it only aims at gauging the number of chromosomes housed in an embryo. As a result, it increases the chance of a healthy pregnancy for couples across age groups.

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  • Preimplantation Genetic Diagnosis

PGD involves the examination of an embryonic cell produced during an in vitro fertilisation (IVF) or intracytoplasmic sperm injection (ICSI) procedure. It is tailored to test for specific genetic conditions (cystic fibrosis, for example) before transferring the embryo to the uterus.

All pregnancies run the risk of a chromosomal abnormality. In fact, it is estimated that half of human fertilisations carry the wrong number of chromosomes, which is a leading cause of miscarriage. The risk of having a pregnancy with a chromosomal abnormality increases as females age. Unlike men, who produce millions of sperm daily, a woman is born with all her eggs already created. The ability of her eggs to produce a healthy child decreases as she gets older. That is why a woman's age is critical when considering the probability of getting pregnant.

Who Is PGS/PGD Suited For?

Any couple undergoing IVF/ICSI can opt for PGS/PGD in order to select a chromosomally normal embryo. If a carefully studied embryo is transferred, the chances of conception increase. Also, the odds of miscarriage and of bearing a child with abnormalities automatically reduce.

  • By and large, PGS/PGD are recommended in the following cases:
  • One or both partners are carriers of chromosomal translocations
  • There is a history of disease in one or both partners
  • A couple has faced failed IVF cycles 3 or more times
  • The female partner undergoing IVF is 38 years or older
  • A couple has faced recurrent miscarriages
  • The male partner has been diagnosed with severe male factor infertility
  • The enablement of a solo embryo transfer, reducing the risk of multiple order births

How Are PGS/PGD Performed?

1.The biopsy routine of a twin PGD/PGS procedure is only open to guests who opt for IVF

2.After the union of the egg and the sperm, embryos are left to develop into cleavage-stage embryos and ultimately, into blastocysts

3.The IVF procedure consists of ovarian stimulation, egg retrieval, egg fertilisation, embryo development and finally, embryo transfer

4.A biopsy is performed at the cleavage stage, typically on day 3 of embryo development. At this point, the embryo is usually composed of 6 to 10 cells

5.Subsequently, a portion of the outer shell of the embryo (zona pellucida) is removed for testing

6.Then, 1 or 2 cells are pulled out of the embryo for further examination.

A biopsy is usually done at the expanded blastocyst stage after the embryo has split into an inner cell mass, a trophectoderm component and a fluid-filled cavity. For this, a small hole is made in the shell of the embryo and several cells that are precursors to the placenta (trophectoderm) are removed for testing.

A genetic evaluation is performed using PCR, FISH or a comparative genomic hybridisation (CGH) technique. Nonaffected or normal embryos are then transferred into the uterus for implantation, potentially resulting in pregnancy.

Disadvantages of PGS/PGD

  • Even with a successful IVF and PGD procedure, pregnancy is not guaranteed after embryo transfer and a term or near-term delivery is also not guaranteed.
  • There is often a need for a frozen embryo transfer cycle after PGD or PGS testing
  • There are costs associated with both the embryo biopsy procedure and the genetic laboratory studies on the cells
  • Guests must seek proper genetic counselling to evaluate the advantages of IVF with PGD
  • Given the inherent limitations of current PGD/PGS technology as well as the potential for misdiagnosis due to embryonicmosaicism, it is recommended that guests undertake prenatal diagnosis (chorionic villus sampling or amniocentesis) even if PGD/PGS is performed
  • Removal of a single cell without breakage or damage is technically difficult and requires skill and experience
  • Damage to an embryo (projected to be 0.1%) may accidentally occur during removal of a cell
  • A relatively large number of eggs or embryos may be found to be abnormal, thus leaving only a few or no healthy embryos for transfer

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