Pre-implantation genetic screening (PGS) in assisted reproduction

September 7, 2018 by admin0

Assisted reproductive technology (ART) is a medical science which helps couples who are unable to conceive, or singles who want to have biological children, live their dream. ART covers a wide spectrum of treatments, In Vitro Fertilization (IVF) being the most common one among others. The IVF process involves extracting eggs from a female body, fertilizing these eggs with male sperms in a laboratory, fructifying the resultant embryo, and implanting it in the female body to effect a pregnancy. However, many women fail to become pregnant after this procedure. Pre-implantation genetic screening (PGS) is a technique that can be used in conjunction with IVF to improve pregnancy rates and also the resultant number of healthy births.

During an IVF procedure, embryos are developed in the laboratory by fertilizing the female eggs with male sperms. An embryo that contains a normal number of chromosomes i.e. 23 pairs of chromosomes in each cell, is called a ‘euploid’ embryo. An embryo that carries an abnormal number of chromosomes is ‘aneuploid’. Examples of aneuploid embryos include those with 18 sets or 21 sets of chromosomes, or maybe a single additional or missing chromosome resulting in a total of 47 or 45 chromosomes instead of the usual 46. Pre-implantation genetic screening (PGS) is a technique used to identify the number of chromosomes present in embryos created through IVF. After PGS, only embryos with a normal number of chromosomes i.e. the genetically healthy embryos are used and furthermore, the good looking ones among these are used for implantation in the uterus. These embryos are considered potentially competent to propagate viable pregnancies and thus help increase the success rate per transfer.

Practitioners in the field of ART recommend PGS of embryos prior to the implantation process, citing financial, physical and most importantly emotional well-being for the hopeful couples or singles undergoing the treatment:

  • PGS gives a statistical basis to the hope of people undergoing treatments, with regards to a achieving a successful pregnancy. For example, if the couple is informed by their practitioner that in their case 20 eggs fertilized into 15 embryos. Of these 15 embryos, only 8 reached the blastocyst stage and could be sent for PGS, and out of those 8 only 3 were reported normal and could be used for implantation. Here, the couple would place their hopes not on the 15 embryos fertilized, but on the 3 that could result in a successful pregnancy. Realistic hopes versus high hopes can make a huge difference to the emotional impact of a failed pregnancy.
  • PGS can help avert the risk of multiple pregnancies in a client, which usually happens when 2-3 embryos are implanted in a single cycle to increase the chance of achieving a pregnancy. The client can achieve success with single embryo transfer cycles for the select number of competent embryos identified through PGS.
  • PGS can help avert the emotional and physical turmoil of a patient, resulting from a miscarriage caused due to implantation of an embryo with abnormal chromosome count, weeks after soaking in the happiness of achieving a successful IVF pregnancy.

However, investigation has also revealed that PGS, though an excellent tool to improve implantation success rates, is not a panacea when it comes to identifying ‘competent’ embryos. There are factors other than numerical chromosomal integrity that influence embryo “competency”, profoundly.

Advancing age of a woman is a key factor contributing to an increase in the incidence of abnormal embryos. For women less than 36 years of age, roughly 50% of the embryos would most likely be competent. Conversely, for a woman in her mid-forties, the chance of an embryo being competent would probably be just around 10-12.5%.

Independent of embryo competency, there are other variables that also determine IVF outcome. These include selection and implementation of protocols for controlled ovarian stimulation (COS), endometrial factors that determine embryo implantation, technical skill of the physician performing embryo transfer etc.

Also, a limiting factor of PGS is the need of embryos to be cultured for 5-6 days until a blastocyst stage is reached, which can then be biopsied and sent for genetic analysis. However, it is well known that some healthy embryos don’t advance to the blastocyst stage. Younger patients have many embryos that will progress to the blastocyst stage, however older patients with fewer eggs and embryos have a lesser opportunity to reach the blastocyst stage.

So in the case of a patient with advanced maternal age who has few embryos available at day 3 (prior to the blastocyst stage) a decision has to be made either in favour of transferring the embryos on day 3 or culture them further with the risk that they don’t make it to the stage where biopsy can be conducted on them.

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