What Is a PGT Biopsy? How Trophectoderm Testing Works

A PGT biopsy is the removal of a small number of cells from an embryo created through IVF, done so those cells can be screened for genetic or chromosomal problems before the embryo is transferred to the uterus. The procedure typically takes place on day five or six of embryo development, when the embryo has reached the blastocyst stage, and involves extracting roughly five to eight cells from the outer layer known as the trophectoderm. It is one of the most debated steps in modern fertility medicine, with strong evidence supporting its value in certain situations and equally strong questions about whether it helps everyone.

What Happens During a Trophectoderm Biopsy

By day five after fertilization, an embryo has differentiated into two distinct cell populations. The inner cell mass will eventually become the fetus, while the trophectoderm forms the outer shell and will later develop into the placenta. The biopsy targets only the trophectoderm cells, leaving the inner cell mass untouched. A laser is used to open a small hole in the zona pellucida, the protective shell surrounding the embryo, and then five to eight trophectoderm cells are gently pulled away, sometimes with additional laser pulses to sever the strand of cells cleanly.¹

One practical complication arises when the inner cell mass herniates, or bulges out, through the opening made in the zona pellucida. In those cases, the embryologist cannot safely biopsy at that site without risking damage to the cells that will form the baby. A workaround involves creating a second opening on the opposite side of the embryo and biopsying trophectoderm cells there instead. A retrospective study of 560 single embryo transfers found that this double-opening strategy did not compromise pregnancy or obstetric outcomes compared to the standard single-opening approach.²

The number of cells retrieved matters. A multicenter study involving over 2,500 biopsies found that the average biopsy yielded about seven cells. Samples with too few cells were significantly more likely to produce inconclusive genetic results, while the number of cells did not vary meaningfully between different embryologists performing the procedure, suggesting the technique is reproducible across practitioners.³

Why Day Five Replaced Day Three

Earlier generations of PGT involved removing one or two cells from a day-three embryo, when it had only six to eight cells total. Removing even one cell at that stage meant taking away a much larger fraction of the embryo. The shift to day-five trophectoderm biopsy was driven by the logic that a blastocyst, which has over a hundred cells, can better tolerate losing a handful. A Cochrane review noted that the invasiveness of biopsy and its timing could affect implantation, pregnancy, and live birth outcomes, though the evidence directly comparing day-three and day-five biopsy for single-gene disorders remains extremely thin.⁴

A large matched study comparing biopsy protocols found that embryos biopsied using a sequential blastocyst hatching and biopsy approach had significantly higher survival rates after thawing, higher clinical pregnancy rates, and higher live birth rates compared to embryos that had undergone a day-three pre-hatching protocol.⁵ The day-three protocol also produced four cases of identical twin pregnancies from embryo splitting, while the blastocyst approach produced none. Today, trophectoderm biopsy at the blastocyst stage is the standard in virtually all clinics offering PGT.

The Mosaicism Problem

The biggest biological limitation of trophectoderm biopsy is that the cells you test are not the cells that become the baby. They are the cells that become the placenta. If every cell in the embryo had the same chromosomal makeup, this would not matter. But embryos are frequently mosaic, meaning some cells carry chromosomal errors while others do not. Mitotic errors during early cell division can scatter abnormal cells unevenly throughout the embryo, driven by factors like disrupted cell-cycle checkpoints and problems with the machinery that pulls chromosomes apart during division.⁶

How often does this cause the biopsy result to disagree with what is actually happening in the inner cell mass? One study using next-generation sequencing found a chromosomal consistency rate between trophectoderm and inner cell mass of roughly 86 to 90 percent, regardless of where on the trophectoderm the biopsy was taken. About 14 percent of embryos showed confined mosaicism, where the trophectoderm and inner cell mass had genuinely different chromosomal profiles.⁷ Another study detected mosaicism in 59 percent of embryos examined, with an exact cytogenetic match between trophectoderm and inner cell mass in only 48 percent of cases. Yet from a clinical standpoint, the picture was more reassuring: when the trophectoderm read as mostly normal, the inner cell mass was also mostly normal about 88 percent of the time.⁸

This discrepancy has real consequences. Some embryos labeled aneuploid (chromosomally abnormal) by trophectoderm biopsy may actually have a normal inner cell mass. Critics of PGT for aneuploidy screening have argued that high mosaicism rates lead to high false-positive rates, meaning normal embryos with full pregnancy potential get discarded.⁹ Re-biopsy studies have confirmed that embryos with certain types of results, specifically mosaic segmental duplications, are a primary source of false positives and can have live birth rates comparable to those of euploid embryos.¹⁰

Can Embryos Fix Their Own Chromosomal Errors?

One of the more remarkable findings in embryology is that some mosaic embryos appear to self-correct. Several proposed mechanisms could explain this, including selective destruction of abnormal cells through programmed cell death, physical exclusion of aneuploid cells from the developing embryo, and shunting of abnormal cells into the tissue that will become the placenta rather than the baby.¹¹ The embryo can also undergo chromosomal rescue events, where a trisomic cell loses the extra chromosome or a monosomic cell duplicates the missing one.

This self-correction capacity is part of why transferring mosaic embryos can still produce healthy babies. A review of more than 100 documented live births after mosaic embryo transfer found reassuring outcomes with no abnormal physical characteristics in the children born.¹² A meta-analysis confirmed that mosaic embryos do implant and lead to live births at lower rates than euploid embryos, but once a mosaic embryo successfully implants, miscarriage rates are comparable between the two groups, suggesting that implantation is the main hurdle.¹³ For patients who have no euploid embryos available, transferring a mosaic embryo after careful counseling is now a recognized option at many clinics.

Does PGT-A Actually Improve Live Birth Rates?

This is where the evidence gets uncomfortable for advocates of universal PGT-A screening. A landmark randomized trial published in the New England Journal of Medicine found that among women with three or more good-quality blastocysts, conventional IVF without any genetic testing produced a cumulative live birth rate that was not inferior to PGT-A. In fact, the PGT-A group had a live birth rate of about 77 percent compared to roughly 82 percent in the conventional group.¹⁴ A separate pilot randomized trial in women aged 35 to 42 similarly found no statistically significant differences in clinical pregnancy rate, live birth rate, or miscarriage rate between PGT-A and morphological selection alone.¹⁵

These results do not mean PGT-A is useless. The value proposition shifts depending on the clinical scenario. PGT-A reduces the number of transfers needed to achieve a live birth, which means fewer failed transfers, fewer miscarriages, and a shorter overall treatment timeline. One cost-effectiveness analysis found that PGT-A reduced treatment time by up to four months for patients with more than one embryo, even though cumulative live birth rates were equivalent once all embryos were eventually used.¹⁶ For patients with very few embryos, though, the calculus can tip the other way: discarding an embryo labeled abnormal that might have self-corrected and led to a healthy pregnancy is a steep price when there are no alternatives waiting in the freezer.

The Preeclampsia Question

A safety signal that has drawn attention is a possible link between trophectoderm biopsy and preeclampsia, a dangerous pregnancy complication involving high blood pressure. One study found that the incidence of preeclampsia was about 10.5 percent among IVF pregnancies that involved PGT compared to 4.1 percent among IVF pregnancies without PGT, representing roughly a three-fold increase in adjusted odds.¹⁷ A systematic review examining multiple studies found conflicting data: some studies replicated this association while others did not, making it difficult to draw a firm conclusion.¹⁸

The biological plausibility is there. The trophectoderm cells removed during biopsy are the precursors to the placenta. Disrupting them could theoretically impair placental development in ways that manifest later as preeclampsia. However, a meta-analysis found no marked difference in the risk of most other obstetric or neonatal outcomes between biopsied and non-biopsied embryos.¹⁹ The preeclampsia signal remains an active area of research, and your fertility clinic should be discussing this potential risk during the consent process.

Freezing and Re-Biopsy Take a Toll

Because PGT results take time, biopsied embryos are almost always frozen through vitrification and transferred in a later cycle. This means the embryo goes through biopsy followed immediately by freezing, then thawing for transfer. One concern is that the timing between biopsy and vitrification matters more than people realize. About 70 percent of blastocysts begin re-expanding their fluid cavity within one to two hours of biopsy. If vitrification happens while the embryo is in the middle of this re-expansion, it may disrupt the delicate balance of water and salt regulation inside the cells, potentially harming the embryo’s ability to re-expand properly after thawing.²⁰

The situation becomes more concerning when an embryo needs to be biopsied and frozen a second time, which happens when a first biopsy is inconclusive or when a mosaic result prompts re-testing. A study comparing embryos that went through the biopsy-freeze cycle once versus twice found that survival after thawing dropped from about 98 percent to 93 percent, and clinical pregnancy rates fell from roughly 54 percent to 31 percent. The combined effect of a second biopsy plus a second freeze was significantly worse than a single round.²¹

How the Genetic Analysis Has Improved

The cells removed during biopsy are only as useful as the technology analyzing them. The earliest PGT methods in the 1990s used fluorescence-based techniques that could examine only a handful of chromosomes at a time. This was replaced by microarray methods that could screen all 23 chromosome pairs simultaneously, and more recently by next-generation sequencing platforms that offer even finer resolution.²² A study comparing outcomes across these technology generations found progressive improvements in pregnancy outcomes as the analytical methods advanced.²³

Comprehensive sequencing approaches can now handle all three types of PGT from a single biopsy sample: screening for aneuploidy (PGT-A), testing for single-gene disorders like cystic fibrosis or sickle cell disease (PGT-M), and detecting structural chromosomal rearrangements like translocations (PGT-SR). One validation study using whole-genome sequencing achieved 100 percent concordance with established methods across all three testing categories.²⁴ This matters practically because patients who need testing for a known genetic condition in their family can simultaneously get aneuploidy screening without a second biopsy or a second laboratory workflow.

Non-Invasive Alternatives on the Horizon

Given the mosaicism limitations and potential risks of biopsy, researchers have been working on non-invasive PGT. The concept is straightforward: embryos naturally shed fragments of DNA into the culture medium they grow in. If you can collect and analyze that cell-free DNA, you could assess the embryo’s chromosomal status without removing any cells at all.

Early results have been mixed. One study detected cell-free DNA in 100 percent of culture medium samples tested and found an overall concordance of about 82 percent with standard trophectoderm biopsy results.²⁵ But another study highlighted a serious problem: non-invasive PGT classified far more embryos as aneuploid than trophectoderm biopsy did, producing 33 false positives. The specificity was only about 51 percent, meaning it would incorrectly flag roughly half of normal embryos as abnormal. Embryos that were euploid by biopsy but aneuploid by non-invasive testing achieved pregnancy rates of 94 percent and live birth rates of 88 percent, underscoring how many transferable embryos would be needlessly excluded.²⁶

More optimistic data have come from studies optimizing the culture conditions. One study looking at frozen-thawed blastocysts reported concordance rates above 90 percent across different culture durations and blastocyst stages, suggesting that with the right protocol adjustments, non-invasive testing could eventually approach the accuracy of biopsy-based methods.²⁷ The technology is not ready for routine clinical use, but it is one of the most actively pursued frontiers in reproductive genetics.

Cost Considerations

PGT-A adds a meaningful expense to an already expensive IVF cycle. Whether it saves money in the long run depends heavily on the patient’s situation. The US-based cost analysis that found PGT-A reduced treatment time also found a cost savings ranging from about $930 to $2,400 per patient for those with more than one embryo, because fewer failed transfers and miscarriages mean fewer additional cycles.²⁸ An Australian analysis of women of advanced maternal age found PGT-A cost-effective at a willingness-to-pay threshold of $50,000 per additional live birth, with over 80 percent probability of being cost-effective from a healthcare-system perspective.²⁹

But a Chinese analysis based on a randomized trial reached a less favorable conclusion, estimating that the average cost per live birth with PGT-A was roughly 17 percent higher than conventional treatment.³⁰ The discrepancy across studies largely comes down to the baseline population, the local cost of IVF cycles, and whether the analysis accounts for the emotional toll of failed transfers and miscarriages, which is real but hard to quantify in economic terms.

Regulation Varies Dramatically by Country

What PGT can be used for depends on where you live. A comparative review of 19 countries found a spectrum ranging from highly restrictive to essentially unregulated.³¹ In Germany and Switzerland, for instance, PGT is permitted only under narrow medical criteria. In the United Kingdom, a centralized authority reviews and approves the conditions that can be tested for. The United States stands at the permissive end: there are no federal or state laws governing PGT specifically, and the decision about what to test for is left to professional guidelines and individual clinics. Non-medical sex selection, which is prohibited in most European countries, is freely available in the US.³²

This regulatory patchwork has fueled cross-border reproductive tourism. Patients from countries with strict PGT rules sometimes travel to the US or other permissive jurisdictions to access testing that is unavailable at home. The ethics get even more complex with the emergence of polygenic risk scores, which attempt to predict traits influenced by many genes simultaneously, like height or disease susceptibility. Some US clinics already offer this type of embryo screening, while European regulators are still debating whether it should be allowed at all.

The Emotional Weight of Waiting for Results

Discussions of PGT tend to focus on success rates and laboratory accuracy, but the psychological dimension deserves attention. A study at a Saudi Arabian fertility center found that roughly 14 percent of women awaiting PGT results had clinically significant anxiety scores, and about 12 percent had clinically significant depression scores. Women with higher education levels, a prior psychiatric diagnosis, or three or more children with congenital problems had significantly higher anxiety.³³ The waiting period between biopsy and results, typically one to two weeks, is an added stressor layered on top of the already intense emotional landscape of IVF. For couples who have experienced recurrent pregnancy loss or who carry known genetic conditions, the stakes of each embryo’s result feel enormous. Clinics increasingly recognize that psychological support should be part of the PGT process, not an afterthought.