Can a biopsy harm the embryo?

PGT-A Results Explained

Preimplantation genetic testing can include Preimplantation Genetic Screening (PGS) which is now known as preimplantation genetic testing for aneuploidy (PGT-A) or Preimplantation Genetic Diagnosis (PGD). PGT-A screens the embryos to find out which have the correct number of chromosomes, and PGD tests the embryos for a certain inheritable genetic diseases such as Cystic Fibrosis or Huntington's Disease. Both PGT-A and PGD use the same biopsy process which involves taking some cells out of the embryo and analysing the DNA.

What is Preimplantation Genetic Testing?

Preimplantation genetic testing is an add-on which is becoming increasing popular because it tests whether embryos have the correct number of chromosomes. This 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.

It may be referred to as preimplantation genetic testing for aneuploidy (PGT-A) or 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.

pipette-tips
PGTA quote

What does it test for?

A healthy embryo should have 23 pairs of chromosomes which contains all of its genes – 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 for females or XY for males. However, genetic errors in eggs and sperm are common and this means that embryos often have extra or missing chromosomes.

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 lead to miscarriage. Unfortunately 50-90% of IVF embryos are chromosomally abnormal and this is the most common cause of miscarriage.

There are a few specific cases where an extra chromosome can create a full-term pregnancy. 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 Biopsied?

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 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.

In the past, embryologists would remove either the polar bodies (small portions of the DNA expelled from the egg during development) or one whole cell (blastomere) when the embryo is around 3 days old. This outdated technique is much more risky as we don’t know whether the biopsied cell would have formed the placenta or the baby and this could potentially be damaging to the foetus. It also has a more significant impact on the development of the embryo, as well as giving a less accurate result. The Human Fertilisation and Embryology Authority (HFEA) do not recommend day 3 embryo biopsy as there is no evidence to show that it has beneficial results. The technique has now advanced substantially and these methods have largely been left behind. Most clinics now take a biopsy at the blastocyst stage on day 5, 6 or 7 from the trophectoderm (outer cells). Biopsying embryos at the blastocyst stage means that more cells can be taken to give a more accurate result and it actually causes less harm to the embryo than older techniques.

day 3 biopsy
polar body biopsy

1-2 days before the biopsy, a small hole will be made in the shell surrounding the embryo called the zona pellucida. This allows several cells to herniate out through the gap overnight. On the day of the biopsy 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 to their potential to form a pregnancy.

blastocyst biopsy

PGT Results

There are three main results from an embryo biopsy:

  • Euploid (chromosomally normal)
  • Aneuploid (chromosomally abnormal)
  • Mosaic (mixture of normal and abnormal cells)

You can read more about biopsy results and what it means for your treatment HERE