Preimplantation genetic testing can include Preimplantation Genetic Screening (PGS) or Preimplantation Genetic Diagnosis (PGD). PGS 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 PGS and PGD use the same biopsy process which involves taking some cells out of the embryo and analysing the DNA.
How is a biopsy done?
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.
On the day before the biopsy (day 4), a laser is used to make a small hole in the shell surrounding the embryo. This allows several cells to herniate out through the gap overnight. The following day, a laser is used to disconnect the herniating cells from the rest of the embryo. Around 4-8 cells are taken and immediately frozen so they can be sent to a specialist lab for analysis. The results can take a couple of weeks to come back so the embryo will also be frozen until it is ready for use.
Embryo biopsy is a technique that requires a lot of skill and experience so only the most senior embryologists can perform it.
Does a Biopsy Harm the Embryo?
There are always risks associated with preimplantation testing because the biopsy procedure is fairly invasive. The less an embryo is handled the better and a biopsy puts it under significantly more stress. It gets exposed to lasers, pulling, and prodding to an extent it would never be faced with naturally in the body. It also means the embryo has to be frozen and thawed while the results are processed which is another round of stress that could potentially be harmful.
Luckily, embryos are surprisingly resilient and large studies have shown that the biopsy-freezing process doesn’t seem to have any serious detrimental effects to their potential to form a pregnancy. However, there is still a small concern that it could result in some long term harm to the baby by potentially causing underlying genetic alterations which increase its susceptibility to diseases further into adulthood. There is still more research needed to determine whether this is a cause for concern or not.
It is also important to ensure that the cells are removed from the trophectoderm - the outer layer of cells which forms the placenta, rather than the inner cell mass which forms the baby. The inner cell mass contains all the stem cells needed to make a fully-formed, healthy baby and if some of these are removed it could have serious detrimental effects to the embryo's development. Sometimes it can be difficult for an embryologist to identify the right cells to remove, for example if the embryo has collapsed. If you are having your embryos biopsied be sure to ask your embryologist how they felt about the procedure and if everything went as planned.
Can a Biopsy be Done on Day 3?
Some clinics are still performing blastomere biopsy where a single cell is taken on day 3 of embryo development.
This technique is much more risky as the embryo is only made up of around 8 cells. At this point it impossible to tell which cells will make the baby and which cells will make the placenta so it is considered too risky to remove one for testing. If it was a critical cell coding for a large part of the baby, removing it could be damaging to the fetus and do more harm than good. When a day 5 biopsy is done, we know which cells will make up the baby and which won't so the risk is minimised.
In addition, only one cell can be taken on day 3 of embryo development whereas on day 5 many cells can be removed without causing any more harm. This larger sample means the results are much more reliable and there is less chance of getting false negative and false positive results.
Even older techniques include polar body biopsy. This is done by removing a small fragment from a mature egg and testing the chromosomes inside. Both polar body biopsy and day 3 biopsy are currently not recommended as there is no evidence to show that they have beneficial results over a day 5 biopsy.