Preimplantation genetic testing (PGT) is performed on day 5 or 6 after egg fertilization, when embryos reach the blastocyst stage. At this point, the embryo has grown to roughly 80 to 100 cells, and a small sample of 5 to 10 cells is removed from the outer layer for genetic analysis. The entire process happens during an IVF cycle, between fertilization and embryo transfer.
Why Day 5 Is the Standard
After an egg is fertilized in the lab, the resulting embryo divides rapidly. By day 3, it has only about 6 to 8 cells, which limits how many can safely be removed for testing. By day 5, the embryo has developed into a blastocyst with two distinct cell groups: the inner cell mass (which becomes the fetus) and the trophectoderm (which becomes the placenta). The biopsy targets the trophectoderm exclusively, leaving the inner cell mass untouched.
Earlier approaches took a single cell from day 3 embryos, but this carried real drawbacks. Removing one cell from a 6- to 8-cell embryo represents a significant portion of the whole organism and appears to reduce the embryo’s ability to implant. A single cell also makes confirmatory testing impossible. Day 5 biopsy, by contrast, removes 5 to 8 cells from a much larger embryo, giving the genetics lab more material to work with while doing less proportional damage. For chromosome screening (PGT-A), blastocyst biopsy is now used in about 98% of cases worldwide.
How the Biopsy Fits Into Your IVF Timeline
The biopsy itself takes only a few minutes per embryo. An embryologist uses a laser to create a small opening in the embryo’s outer shell, then gently removes a cluster of trophectoderm cells. Labs generally aim for 7 to 8 cells per sample, which provides enough genetic material for reliable analysis.
Not every embryo qualifies for biopsy. The blastocyst needs to be expanding or fully expanded, with reasonably good-quality cell structures in both the inner cell mass and the trophectoderm. Poorly developed embryos or those that are still compacting on day 5 may be given an extra day to develop, with biopsy on day 6 instead. Some clinics have recently begun including lower-quality blastocysts for biopsy, but the standard practice still favors embryos graded at a certain quality threshold.
After the cells are removed, they are sent to a genetics laboratory while the embryo is typically frozen through vitrification. Survival rates after biopsy and freezing are excellent, reaching 98 to 100% in published data. The embryo stays frozen until results come back and a transfer cycle is planned.
Waiting for Results
Genetic results usually take 7 to 14 days, though turnaround can stretch longer if the lab is busy or the analysis is complex. During this time, your biopsied embryos remain safely frozen.
Because of this waiting period, most PGT cycles use a freeze-all approach: every embryo is vitrified after biopsy, and you return for a frozen embryo transfer in a later cycle. This is the most common strategy worldwide. However, advances in sequencing technology have made overnight PGT-A results possible in some labs, which opens the door to fresh transfers without freezing. Clinical studies show pregnancy and live birth rates are similar between fresh and frozen transfer after PGT, so the choice comes down to logistics and lab capability rather than a medical advantage either way.
Timing Differences Between PGT Types
There are three types of PGT, and while all three can be performed at the blastocyst stage, the shift to day 5 biopsy has happened at different speeds for each.
- PGT-A (aneuploidy screening) checks for the correct number of chromosomes. This is the most common type, often recommended for people with recurrent miscarriage or unsuccessful IVF transfers. Nearly all PGT-A testing now uses blastocyst biopsy.
- PGT-M (monogenic/single gene disorders) screens for specific inherited conditions like cystic fibrosis or sickle cell disease. The transition to day 5 biopsy has been slower here. As recently as 2018, about 65% of PGT-M cases still used day 3 biopsy, though that number continues to decline.
- PGT-SR (structural rearrangements) looks for chromosomal translocations or inversions. Like PGT-M, it has been slower to adopt blastocyst biopsy, with about a third of cases using day 5 biopsy in 2018. Because this method analyzes only a few trophectoderm cells, confirmatory testing through chorionic villus sampling or amniocentesis during pregnancy is typically recommended.
The biopsy day and technique are identical across all three types. The difference lies in what the genetics lab looks for once they receive the cells, and some clinics now offer combined testing that screens for aneuploidy and a specific genetic condition simultaneously.
Preparation Before the Biopsy
PGT-M and PGT-SR require advance preparation that starts weeks or even months before your IVF cycle. For single-gene testing, the genetics lab needs to build a custom test (called a probe or linkage panel) specific to your family’s mutation. This involves collecting DNA samples from you, your partner, and sometimes other family members. Plan for this setup phase to take 4 to 8 weeks before your eggs are even retrieved.
PGT-A requires no advance genetic workup. It uses standardized chromosome screening technology, so the lab can process your embryo samples without prior preparation beyond the standard IVF cycle.
Noninvasive Alternatives on the Horizon
Researchers are developing a way to perform PGT without physically removing cells from the embryo at all. The approach, called noninvasive PGT (niPGT), analyzes DNA that embryos naturally release into their surrounding culture medium during the blastocyst stage. After 24 hours of culture, the spent medium is collected and sequenced.
Early results are promising. One study published in the Proceedings of the National Academy of Sciences found that niPGT-A had higher concordance rates with the embryo’s true chromosome status than traditional trophectoderm biopsy, partly because it captures DNA shed from both the inner cell mass and the outer layer rather than sampling only one region. This could reduce errors caused by mosaicism, where different parts of the same embryo have different chromosome makeups. The technology is not yet widely available in clinical practice, but it represents a meaningful shift in how and when testing could be done, potentially eliminating the biopsy step entirely.