An overview of PGS/PGT-A testing

An overview of PGS/PGT-A testing

So it’s the Christmas holidays and as a former embryologist the one thing that feels most like Christmas is PGS testing. Or PGT-A (whatever you want to call it!).

You do a biopsy on an embryo, you package it into a nice little cooler, and send it off to the testing lab where it’s tested. And a week or so later you get the results back! And no one really knows what’s inside.

So PGS testing is a technique that lets us take a closer look at what’s happening inside the cells of an embryo. And why do we care about this? Because we want to know how many chromosomes are in there.

Chromosomes are how our cells package DNA. There’s just so much of it! If you were to stretch it out, it would be about 6 feet long, which coincidentally is my height. Hmm. Anyways this is pretty impressive when you consider that you have 6 feet of DNA in every cell in your body. And you have about 30 trillion cells in your body!

So each cell has 46 chromosomes. Half come from the egg and half from the sperm. And these 46 chromosomes are like chapters in a cookbook with all kinds of recipes on how to put the human body together. They’re not called recipes though, they’re called genes. And having the right amount of them is important. The same way you wouldn’t want to add too much sugar to a recipe, or too little.

So having 46 chromosomes is important. You don’t want to have more, and definitely don’t want to have less. It even has a name that sounds scary: aneuploidy. Euploidy is having the right number of chromosomes. And everyone, including most of the cells in your body want that!

So what happens if you have the wrong number of chromosomes?

It’s not for sure, but some evidence points to increased miscarriage. Women who are older might be at increased risk for this too. They might have errors in their eggs that predispose them to aneuploidy which might explain why it’s harder to get pregnant with advanced age. This leads to a reduced chance of getting a euploid embryo with increased age (reference:

  • ≤ 35: 61%
  • 36: 56%
  • 37: 51%
  • 38: 46%
  • 39: 41%
  • 40: 37%
  • 41: 32%
  • 42: 27%
  • 43: 22%
  • 44: 17%
  • 45: 12%

So PGS testing is all about figuring out how many chromosomes the cells of the embryo have. Except you don’t get to see the whole picture. You’re only looking at a tiny piece of the embryo. So we’re taking a leap and saying that the tiny biopsy we take corresponds to the rest of the embryo.

That’s not always the case, and sometimes this can lead to false positives: embryos that test aneuploid from the biopsy, but other parts of the embryo are actually euploid. Some believe that a much larger piece of the embryo needs to be taken for a biopsy in order for it to be statistically accurate. There’s even evidence that biopsying might damage the embryo!

Then there’s the issue of mosaics, where some cells are euploid and some are aneuploid, and no one really knows what to do with them. Patients want to transfer them, because in some cases that’s all they have, and some doctors are reluctant because we don’t know if the good or the bad cells will dominate (see my blog on mosaics here

This has some critics of PGS testing up in arms about the whole thing.

So does PGS testing work? Yes and no. Recent data suggests that for women who are over 35 there might be a positive benefit in transferring a euploid embryo. For women less than 35, you might be better off with conventional embryo grading. That’s right. Euploids don’t always work, and in some cases they’re no better than just standard embryo grading (Reference: There’s a lot of studies that look at the efficacy of PGS testing and honestly it’s pretty controversial.

Added to this, for some anyways, is a large expense. PGS testing is not cheap. It can cost thousands of dollars on top of IVF, and to some people this is a surprise. As a former embryologist, I remember a few patients being shocked that they had to pay for this separately. The biopsy itself is done in the IVF lab, but the genetic testing (DNA sequencing) is done with an outside lab.

Want to learn more about PGS testing? Visit my comprehensive blog that goes into its history, how it’s done, what it means, the statistics and the controversy by visiting

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