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Nov 11, 2026, 15:19

Beyond Trophectoderm Biopsy: Technical Strategies, Mosaicism Interpretation and Quality Indicators in PGT-A Laboratories

“A practical evidence-based perspective for clinical embryologists and IVF laboratory teams.

Introduction

Preimplantation genetic testing for aneuploidy (PGT-A) is a multistage diagnostic process. Its reliability depends on embryo biopsy, sample handling, whole-genome amplification (WGA), sequencing and clinical interpretation. Trophectoderm (TE) biopsy at the blastocyst stage is now the predominant approach, but a TE result remains a sample-based inference rather than a complete chromosome analysis of every embryonic cell.

Current ESHRE recommendations emphasise standardised biopsy, contamination prevention and traceability, while ASRM cautions against treating PGT-A as a universal screening requirement for every IVF patient (ESHRE, 2020; ESHRE, 2026; ASRM, 2024).

Why is blastocyst biopsy preferred, and what does self-correction evidence show?

Cleavage-stage biopsy generally removes one blastomere from a Day 3 embryo, often containing six to eight cells. Removing one cell represents approximately 12.5–16.7% of the observed cell number; removal of two cells increases the proportional loss and is discouraged. At this stage, blastomeres have not committed firmly to the inner cell mass (ICM) or trophectoderm (TE), and a single cell may not represent an embryo containing mitotic chromosomal heterogeneity.

The clinical concern is supported by a randomized, paired trial in which sustained implantation resulting in live birth occurred in 30% of biopsied cleavage-stage embryos versus 50% of paired unbiopsied controls. Outcomes for biopsied and control blastocysts were similar (51% versus 54%) (Scott et al., 2013).

Blastocyst biopsy, usually performed on Day 5–7, provides approximately 5–10 TE cells for analysis while leaving the ICM physically intact. ESHRE identifies blastocyst biopsy as current standard practice. Day 3 biopsy is therefore best described as less commonly used and limited, rather than universally obsolete; selected clinical or regulatory circumstances may still influence biopsy strategy (ESHRE, 2020; ESHRE, 2026).

Embryonic self-correction refers to proposed processes that may alter the relative abundance or distribution of abnormal cells during development. In 125 human blastocysts, Regin et al. (2024) associated greater chromosomal imbalance with p53 pathway activation, autophagy and apoptosis. These findings support cellular stress responses to aneuploidy, but do not prove that all abnormal cells are eliminated or that an embryo becomes uniformly euploid.

Preferential allocation of abnormal cells to TE and normal cells to ICM, as well as differential proliferation of euploid and aneuploid cells, have been proposed. Evidence for predictable lineage selection in human embryos remains limited. Single-cell sequencing research has also identified extensive cell-to-cell heterogeneity in human blastocysts (Xie et al., 2024). Consequently, a TE result cannot establish the chromosomal constitution of the ICM, and self-correction must not be presented as a guaranteed biological outcome.

How should biopsy, laser and tubing workflows be standardised?

ESHRE recommends changing the biopsy pipette for each blastocyst; it also permits thorough rinsing as an alternative when the laboratory has verified that its procedure prevents cross-contamination (ESHRE, 2020). In some high-throughput clinical laboratories, a cohort-dedicated biopsy pipette is used for embryos from the same patient, with a validated sterile-media flushing/rinsing sequence between embryos.

This is an operational option—not a blanket ESHRE endorsement—and should be described as locally validated only when supported by documented carry-over testing, negative controls, predefined acceptance criteria and ongoing audit. A same-patient cohort is not genetically homogeneous: embryos may have different chromosomal complements, so patient identity alone does not remove the risk of sample carry-over.

A validated cohort workflow may reduce pipette-change interruptions, handling time outside controlled culture conditions and repeated micromanipulator realignment; any claimed reduction in temperature or pH exposure should be confirmed by the laboratory’s own workflow assessment.

A new sterile biopsy pipette is mandatory when switching to a different patient, and it should also be replaced immediately after cell lysis, visible debris or suspected contamination. During washing and tubing, a new sterile transfer/stripping pipette must be used for each individual embryo, with at least two washes and minimal medium carry-over (ESHRE, 2020).

Infrared diode lasers near 1.48 μm are used for zona pellucida opening. Their effect is photothermal, and the outcome depends on wavelength, power, pulse duration, focus, target geometry and number of exposures. ESHRE recommends a guided non-contact laser and adherence to manufacturer specifications. A pulse duration of 200–300 μs is not a universal standard. Taylor et al. (2010) compared 604, 708 and 1,010 μs pulses in cleavage-stage biopsy and found no difference in Day 5 embryo quality; these data do not establish an optimal setting for modern TE biopsy.

Tubing should occur in a designated clean area with strict sample identification and contamination controls. Biopsied cells should be washed at least twice using sterile transfer pipettes; the co-transferred medium should be kept below 1 μL. At least one negative control per relevant collection or washing buffer is recommended. If cell lysis or visible sample loss occurs, the potentially contaminated pipette should be discarded (ESHRE, 2020).

How should mosaic PGT-A results and laboratory quality indicators be interpreted?

In NGS-based PGT-A, mosaicism is inferred from an intermediate copy-number signal in amplified DNA from a small TE sample. It is not a direct count of abnormal cells in the whole embryo. Laboratories use assay-specific thresholds; a 50% boundary may be used to distinguish lower- and higher-range categories, but it is a reporting convention rather than a universal biological cut-off. Reports should state validated thresholds and analytical limitations (ASRM, 2023).

Mosaic embryos should not automatically be discarded. Where no euploid embryo is available, transfer may be considered following individualised counselling. Relevant considerations include the chromosome involved, segmental versus whole-chromosome findings, mosaic range, number of chromosomes involved, assay limitations and patient preferences. Mosaic percentage alone cannot reliably predict fetal risk.

Routine re-biopsy solely to confirm or overturn a mosaic result is not justified. A second TE sample may differ because of spatial heterogeneity and still does not establish the ICM or whole-embryo chromosome complement. Re-biopsy may be considered selectively after failed, incomplete or inconclusive diagnosis, subject to local policy and discussion of additional warming, biopsy and refreezing (ESHRE, 2020; ASRM, 2023).

Prenatal counselling should explain that NIPT analyses predominantly placental cell-free DNA and is a screening test. CVS also samples placental tissue, whereas amniocentesis analyses fetal-derived cells and provides a more direct assessment of fetal chromosome status. Neither test detects every possible mosaic cell population; decisions should be individualised with genetics and obstetric specialists (ASRM, 2023).

A PGT-A quality programme should monitor biopsy completion, biopsy-related degeneration, sample loss, contamination, WGA failure, no-result rate, re-biopsy, post-warming survival, identification discrepancies and operator-specific performance. Clinical outcomes—including implantation, miscarriage and live birth—should be reviewed with the clinical and genetics teams.

Targets such as a no-result rate below 1% and post-warming survival above 95% may be adopted as internal improvement goals, but they are not universal ESHRE or ASRM accreditation thresholds. Each centre should define denominators, exclusions, audit intervals and action limits. Zero tolerance should apply to patient or embryo identification errors.”

Written by Saadat Hassan
Senior Clinical Embryologist/IVF and Embryology
BSc (Hons) Medical Laboratory Technology
King Edward Medical University, Lahore, Pakistan