Pre-Implantation genetic testing for aneuploidy (PGT-A) - mosaic embryo results
Pre-Implantation genetic testing for aneuploidy (PGT-A) - mosaic embryo results
Dr. Pokpong Pansrikaew
Dr. Pokpong Pansrikaew

My results indicate my embryo is mosaic, what does this mean?

Chromosomes are the packages of genetic information within our cells. Usually our cells contain 46 chromosomes (23 pairs). We receive half from the egg and half from the sperm.

Having too many or too few chromosomes (aneuploidy) may result in health problems and is the most common reason for embryos to either not implant, or to miscarry.

What is Mosaicism?

When an embryo is categorised as mosaic, this means that the sample biopsied has been identified as having some cells with the standard number of chromosomes (‘euploid’) and some cells with the incorrect number of chromosomes (‘aneuploid’).

How is mosaicism detected in PGT-A?

PGT-A involves testing the embryo for aneuploidy before it is transferred into the uterus. An embryo biopsy performed at the blastocyst stage (around day 5) collects around five to ten cells from the ‘trophectoderm’. These are the cells around the outside of the embryo that go on to form the placenta. If a proportion of the cells tested show

a chromosome change has been detected, the embryo is classed as mosaic. Literature indicates that approximately 4 per cent of embryos tested at day 5 or 6 are mosaic for chromosomal changes.

Can I transfer a mosaic embryo?

Yes. A mosaic embryo can be considered for transfer only after a genetic counselling

consultation to discuss the specific findings in your embryo. The genetic counsellors may be able to provide you with additional information about the possible outcomes, based on the proportion of cells affected and the specific chromosome involved.

Why is it okay to transfer a mosaic embryo?

The trophectoderm cells that are tested form the placenta. The cells of the embryo which go on to form the foetus (baby) come from the inner cell mass (ICM) and are not biopsied and tested. Mosaicism detected by PGT-A may represent cells in both the trophectoderm and the inner cell mass, or may be confined only to the trophectoderm.

Therefore, if the affected cells are confined to the placental cells, literature shows there is a reasonable chance of the embryo resulting in a liveborn healthy baby.

If the chromosome changes were in enough cells of the embryo, the embryo would likely not implant or may miscarry early in a pregnancy. However, there is also a chance of an ongoing pregnancy or the birth of a baby with health problems, such as developmental, physical and intellectual disabilities.

Based on the current literature about the chromosomes involved and the proportion of cells affected, we are able to categorise and provide approximate statistics of your

mosaic embryo resulting in an ongoing healthy pregnancy. Knowing this information assists us to prioritise your embryos for transfer, and is one of several factors your fertility specialist will use in determining the most suitable embryo for transfer.

Do I need to have further testing during pregnancy?

There is a small chance that the chromosome change in your mosaic embryo is present in an ongoing pregnancy. Furthermore, as PGT-A is not 100 percent accurate, the following tests are available to any couple following an embryo transfer (regardless of the PGT-A result/category):

  • Detailed ultrasounds to look at the development of the baby
  • Diagnostic testing via an amniocentesis – which is the most accurate way to test for any chromosome changes in the pregnancy. Amniocentesis is associated with a small chance of miscarriage (~1 in 200 or 0.5%)
  • Non-invasive prenatal testing (NIPT) –This is a chromosome screening test on maternal blood from around 10 weeks of a pregnancy. If you receive a ‘low-risk’ result, this provides reassurance that the baby most likely has standard chromosomes.

Any pregnancy established from the transfer of a mosaic embryo needs to be monitored closely,

and we recommend discussion with your fertility specialist and team managing your pregnancy about the most appropriate prenatal testing and screening for your specific situation.

Regardless of your prenatal testing result, we also recommend a third trimester growth ultrasound to assess the growth of the baby. There may still be a chance of growth issues due to placental dysfunction related to the chromosome change affecting some of the placental cells.

Mosaic embryos have been well documented to result in the births of healthy children. The decision to transfer an embryo with mosaicism is made with the understanding that there may be an increased risk of an adverse pregnancy

outcome. Your treating IVF specialist will support you in making the decision to transfer such an embryo and will help answer any questions you may have.

Please refer to the PGT-A patient information sheet for additional information regarding general PGT-A risks and benefits.