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Deep Dive

What is PGS & the Theory Behind It?

PGS is a procedure in which doctors test some cells from an embryo with hopes of better understanding the genetic makeup of that embryo and, eventually, making a correct determination of the best embryo to transfer.

What the Procedure Entails


In order to have cells to perform testing on, an embryologist must biopsy, or cleave, a few cells off of the outer layer of an embryo. Cells are removed from the part of the embryo called the trophectoderm, which eventually becomes the placenta. The assumption of PGS is that the cells in this outer layer are representative of those in the “inner cell mass” that eventually become the fetus.

This biopsy is performed on blastocyst stage embryos, so usually on day five and sometimes on day six. In an earlier era of PGS testing, the biopsy happened on day three when the embryo had far fewer cells, but that’s no longer standard of care due to a high risk of harming the embryo.

Performing an embryo biopsy takes a lot of skill – embryos are still very small and fragile, and cleaving just the right amount of cells from the right place is something that embryologists must master and practice over a period of time.

Once the five – seven cells have been taken, your clinic will most likely freeze your embryo and send the small sample to an outside “reference lab” to be analyzed. This reference lab will use a handful of possible technologies to analyze the sample – the most common platforms are “Next Generation Sequencing” or “Array CGS.” The goal of the analysis is to count the number of chromosomes present in the sample’s cells.

The Goal of PGS


The goal of PGS is to help your doctor and embryologist select the very best embryo to transfer. Embryo selection hinges on (1) finding the embryo with the very best chances of a healthy live birth and (2) avoiding transfers using embryos that would likely lead to miscarriage or a baby born with health issues.

The Power of Chromosomes


So what is helping doctors understand which embryo is best? PGS looks at whether each cell tested has the correct chromosomes, and in the right quantities.

Chromosomes are basically the genetic instructions that tell our cells how to function – we get one set of 23 chromosomes from each parent for a total of 46 chromosomes. Cells that have the proper number of chromosomes are called “euploid,” and those with the incorrect number of chromosomes are “aneuploid.”

Having extra or missing chromosomes is catastrophic. Usually, for a pregnancy to proceed at all, the embryo being carried must have the correct number of chromosomes. If, instead, an embryo is aneuploid, a pregnancy either never starts because the embryo doesn’t implant in the uterus, ends in miscarriage, or results in the birth of a child with a health issue such as Down’s Syndrome.

The primary goal of PGS testing is to identify which embryos are euploid, so that a doctor can transfer euploid embryos more often, getting a better live birth rate and a lower miscarriage rate.

This means that every transfer has a higher probability of actually working – you’ll likely hear clinics talk about 60 – 70% success rates when they transfer PGS tested embryos. Because the rate of aneuploidy increases significantly with age and is a primary cause of the age-related decline in fertility, success rates from transfers become very similar across age brackets when PGS is used to screen out aneuploidy.

PGS & Twins


An ancillary benefit in this increased confidence doctors and patients have in a live birth following a transfer of a PGS-tested embryo is that there’s higher willingness to do an elective single embryo transfer. This has brought down the rate of twin pregnancies, and the related complications for mothers and babies.

The Bottom Line


After learning about PGS, it might seem like a no brainer – simply screen embryos, determine which are chromosomally normal before transfer, and increase live birth rates. But it’s not cut and dry, and PGS is one of the most hotly contested areas within fertility treatment today. First, it’s not free. And second, the procedure itself has plenty of detractors in the field.