Blog
Sep 21, 2026
When several embryos are available during IVF treatment, one of the most important decisions is determining which embryo should be transferred first. Traditionally, this decision is largely based on the embryo’s appearance and development in the laboratory. Chromosome testing can provide additional information that may help embryologists and doctors assess the potential of individual embryos and establish their transfer order.
A typical human embryo should have the correct number of chromosomes. An embryo with an extra or missing chromosome is described as aneuploid.
Chromosomal abnormalities are common in human embryos and become more frequent as maternal age increases. The potential benefit of PGT-A is particularly relevant when several embryos are available and for women over 38 years of age, in whom abnormalities in chromosome number occur more frequently.
Aneuploidy can prevent an embryo from implanting, lead to an early miscarriage or, in some cases, result in a chromosomal condition in an ongoing pregnancy. Chromosomal abnormalities are an important cause of early miscarriage.
Testing does not change the chromosomes of the embryo. Instead, the information can help in deciding which embryo to prioritise for transfer.
PGT-A (preimplantation genetic testing for aneuploidy) examines the number of chromosomes in an embryo. It can provide additional information about the embryo’s likelihood of establishing a pregnancy and developing further, and the result can be used together with embryo morphology and development when determining the order in which embryos are considered for transfer.
PGT-A is not a test for individual genetic diseases or gene mutations. Its purpose is specifically to assess chromosome number.
Following egg collection, the eggs are fertilised using IVF or ICSI and the resulting embryos are cultured in the laboratory.
For conventional PGT-A, embryos are cultured until the blastocyst stage on day 5, 6 or 7. A small sample, typically around 5-8 cells, is taken from the outer part of the blastocyst. This is known as an embryo biopsy. The embryo itself is then vitrified and stored while the sample is analysed by a genetic laboratory.
The cells taken during the biopsy come from the part of the blastocyst that will later contribute mainly to the placenta rather than from the inner cell mass that develops into the fetus.
The result is then considered together with the embryo’s morphology and development when deciding which embryo should be considered first for transfer.
No embryo chromosome test is 100% accurate. A biopsy contains only a small sample of cells from the embryo. The biopsy result corresponds to the embryo’s chromosome status with over 95% accuracy, although mosaic cell populations may occasionally be missed or overrepresented in the sample.
Overall test accuracy depends on the quality of the sample and the limitations of the method, and at best an accuracy above 95% can be achieved.
For this reason, PGT-A reduces uncertainty but does not eliminate it.
niPGT-A means non-invasive PGT-A. Instead of removing cells from the embryo, niPGT-A analyses genetic material found in the culture medium surrounding the embryo. In this method, the embryo is washed during culture to remove maternal cells surrounding the embryo. The embryo then continues developing until the blastocyst stage. A sample of the spent culture medium is subsequently collected without taking cells directly from the embryo.
The result helps identify embryos that appear to have a normal chromosome number and embryos in which chromosome number abnormalities have been detected.
This information can improve embryo prioritisation, but PGT-A should not be seen as a simple “normal or abnormal” test in every case.
For example, an embryo may be reported as mosaic. Mosaicism means that the analysed sample contains a mixture of cells with normal and abnormal chromosome findings. Mosaic findings occur in approximately 5% of embryos examined by biopsy at Ovumia. Mosaic embryos may still be considered for transfer in selected situations, although pregnancy rates are generally somewhat lower than with embryos without a detected mosaic finding. Decisions are therefore made individually.
Sometimes only part of a chromosome is missing or present as an additional copy. These segmental or partial chromosome abnormalities may also require individual interpretation rather than an automatic decision regarding embryo transfer.
However, PGT-A does not guarantee pregnancy or the birth of a healthy child.
As with any IVF laboratory procedure, there are important limitations.
An IVF cycle may produce no blastocysts suitable for testing. Occasionally no result can be obtained from a sample, and it is also possible that none of the tested embryos are considered suitable for transfer based on the findings.
With biopsy-based PGT-A, the procedure usually does not affect the viability of good-quality blastocysts, although lower-quality blastocysts may be more sensitive to biopsy.
Embryos are normally frozen after sampling and transferred later in a frozen embryo transfer cycle. More than 95% of blastocysts survive freezing and thawing.
PGT-A is an embryo-selection tool, not a guarantee that a future child will be free of genetic or chromosomal conditions. Chromosome testing can reduce the risk of transferring an affected embryo but does not guarantee the birth of a healthy child.
PGT-A therefore does not replace pregnancy monitoring, prenatal screening or diagnostic testing recommended by healthcare professionals.
The value of PGT-A depends on the individual treatment situation.
It may be particularly useful when several blastocysts are available and there is a meaningful choice between embryos. Age is also an important consideration because chromosome-number abnormalities become more common in embryos as maternal age increases.
However, PGT-A is not automatically beneficial for every patient. The decision should be made together with the fertility doctor and embryology team after considering factors such as age, treatment history, number of embryos and the individual goals of treatment.
Ovumia offers embryo chromosome testing as part of selected IVF treatments. Depending on the clinic and treatment plan, testing may be performed using conventional embryo biopsy or non-invasive sampling of the embryo culture medium.
Your fertility doctor and embryologist can explain which approach is available and whether chromosome testing could provide useful additional information in your individual treatment.
Patients undergoing PGT-A have a separate counselling session with an embryologist. During the session, the procedure and key aspects of the test are explained, and the patient has the opportunity to discuss the test and ask questions directly with an expert.
