The Patient's Guide to Embryology

Preimplantation Genetic Testing for Aneuploidy (PGT-A)

PGT-A can identify embryos that are more likely to produce a healthy pregnancy and eliminate the ones that are abnormal.

What is PGT-A?

Preimplantation genetic testing for aneuploidy (PGT-A) is an optional add-on treatment which is becoming increasing popular in IVF. It tests whether embryos have the correct number of chromosomes which helps to identify the ones that are more likely to implant and go on to produce a healthy pregnancy and eliminates the ones that are abnormal.

You may also hear PGT-A referred to as preimplantation genetic screening (PGS). There are also other variations known as PGD, PGT-M and PGT-SR which test for specific genetic diseases, but these are only used in special circumstances where required.

What does it look for?

A healthy embryo should have 23 pairs of chromosomes – one set from the egg and one set from the sperm. 22 of the pairs are simply numbered 1-22 and 1 pair are the sex chromosomes; either XX in female embryos or XY in male embryos. However, genetic errors in eggs and sperm are common and this means that embryos often have extra or missing chromosomes (known as aneuploidy). PGT-A checks all 23 pairs of chromosomes to determine whether embryos are abnormal (aneuploid) or normal (euploid).

In the majority of cases, an embryo with extra or missing chromosomes can’t make a healthy pregnancy. It will either cause the embryo to stop developing before it implants, or if the embryo does implant it would likely lead to miscarriage. Unfortunately a huge proportion of IVF embryos are chromosomally abnormal and this is the most common reason IVF fails as well as the most common cause of miscarriage. There are a few specific cases where an extra chromosome can create a full-term pregnancy. For example, an extra copy of chromosome 21 means the baby will have Down Syndrome, an extra copy of chromosome 13 causes Patau’s Syndrome, and an extra chromosome 18 causes Edward’s Syndrome. These are syndromes which are commonly tested for during pregnancy.

How are embryos tested?

To test an embryo’s DNA, cells need to be removed and sent to a specialist lab to be analysed. The best time to take a biopsy is 5-6 days after fertilisation. At this point the embryo reaches the blastocyst stage and is made up of hundreds of cells, so taking a few for testing has little/no effect on the embryo’s development.

1-2 days before the biopsy, a small hole is made in the shell surrounding the embryo called the zona pellucida. This allows a group of cells to push themselves out through the gap as the embryo expands. Approximately 4-8 of these cells are removed using a tiny pipette. The procedure is very fiddly, it requires a lot of skill and only the most experienced embryologists can perform it.

The cells are immediately frozen and sent to a genetics lab to be analysed. The results can theoretically be generated on the same day but many genetics labs need to batch samples meaning that it can often take several weeks. The biopsied embryo will also be frozen until the results come back. Embryos are surprisingly resilient and the biopsy-freezing process doesn’t seem to have any serious detrimental effects, although the long term effects are still unknown.

PGT-A Results

There are 4 main results that can be generated from an embryo biopsy:

  • Euploid (chromosomally normal) – these embryos can be transferred and have a higher chance of success than untested embryos
  • Aneuploid (chromosomally abnormal) – these embryos cannot be transferred
  • Mosaic (mixture of normal and abnormal cells) – these embryos can potentially be transferred depending on what percentage of cells are abnormal and what the abnormality is
  • No result – in rare cases a result can’t be generated because of a problem with the sample such as contamination or not enough DNA is available

What are the pros and cons of PGT-A?


  • Reduces the amount of time it takes to get pregnant as avoids transfers with embryos that are unhealthy
  • Lower chance of miscarriage
  • Each embryo transfer has a higher chance of success
  • Avoids the need to transfer more than one embryo so reduces the risk of twins
  • Embryos are frozen while the results are generated which gives the body chance to recover from the hormone treatment and egg collection


  • Expensive – requires the additional costs of embryo biopsy, genetic testing, embryo storage and frozen embryo transfer
  • Results are not always accurate/conclusive
  • Doesn’t increase the chance of getting pregnant, only reduces the time to getting pregnant
  • The biopsy is an invasive procedure and can potentially damage the embryo
  • There is a risk that the IVF cycle will be cancelled if there are no embryos suitable for biopsy/transfer

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