Trophectoderm biopsy & 24-chromosome copy number screening for embryo selection
By Day 5–6 of in-vitro culture, a fertilized embryo has progressed from a compact morula into a blastocyst — a fluid-filled structure with two distinct cell lineages. This architecture is what makes trophectoderm biopsy possible without directly sampling the cells that will become the fetus.
The blastocyst has three visible structures under the microscope:
• Zona pellucida — the acellular glycoprotein shell surrounding the embryo, which the blastocyst must expand against and eventually hatch from • Blastocoel — a fluid-filled cavity created by trophectoderm ion pumps that draw water inward, expanding the whole structure • Trophectoderm (TE) — a single-cell-thick epithelium lining the inside of the zona, comprising the majority of blastocyst cells; it will form the placenta, chorion, and other extra-embryonic membranes • Inner cell mass (ICM) — a compact cluster of pluripotent cells attached to the inside of the TE at one pole; it will form the fetus itself and the amnion
Embryologists grade blastocysts using the Gardner system: an expansion score (1–6, hatching = 5–6) followed by two letters grading ICM and TE cohesion/cell number (A = excellent, C = poor), e.g. "4BB" or "5AA".
Because TE and ICM are already segregated by Day 5, a biopsy taken exclusively from the TE samples the placental lineage rather than the cells that form the fetus — the biological basis that makes PGT-A biopsy feasible.
Earlier generations of preimplantation testing biopsied a single blastomere from a Day-3 cleavage-stage embryo — before TE/ICM lineages separate — removing 1 of only ~8 totipotent cells and measurably reducing implantation potential.
Day 5–6 TE biopsy instead removes several cells from a pool of 150–250 that are already committed to become placenta, leaving the ICM (and the vast majority of the embryo) untouched. This lineage segregation is what allows 5–10 cells to be sampled with minimal impact on developmental potential — though it is not perfect, since the TE and ICM both derive from the same zygote and can occasionally diverge genetically after fertilization (confined placental mosaicism).
TE biopsy requires a sufficiently expanded blastocyst so that TE cells are herniating or easily accessible through the zona — this typically means waiting until Day 5, 6, or occasionally Day 7 of culture.
Because NGS-based analysis takes several days, essentially all PGT-A cycles today use a "biopsy-and-freeze" (freeze-all) protocol: every biopsied blastocyst is vitrified immediately after biopsy and stored while results are pending, rather than being transferred fresh. Once results return, a euploid embryo is selected and transferred in a subsequent frozen embryo transfer (FET) cycle, which also allows the endometrium to be prepared independently of ovarian stimulation.
Trophectoderm biopsy is performed under high magnification using a micromanipulation rig: a holding pipette stabilizes the blastocyst, a fine glass biopsy pipette aspirates a small group of TE cells, and a non-contact infrared laser severs the intercellular bridges — all within a few minutes per embryo.
1. The blastocyst is held gently in place with a blunt holding pipette using light suction 2. A region of TE cells distant from the ICM is identified and oriented toward the biopsy pipette 3. A brief series of infrared laser pulses is fired at the zona pellucida to create a small opening, and/or at the intercellular junctions between TE cells to loosen them — the laser is non-contact and thermally localized, minimizing collateral damage 4. The biopsy pipette gently aspirates 5–10 herniating or loosened TE cells through the opening 5. A final laser pulse (or gentle pulling motion) severs the cellular bridge, detaching the biopsied cell cluster from the embryo 6. The biopsied cells are placed into a designated tube for whole genome amplification; the embryo is vitrified within minutes
Laser pulses are fired in short bursts (milliseconds) and are strictly non-contact — the laser never physically touches the embryo — which minimizes thermal spread and mechanical trauma compared to older mechanical or chemical (acid Tyrode's) zona-breaching methods.
Skilled embryologists deliberately orient the blastocyst so that the biopsy site is on the opposite pole from the visible ICM. Because the ICM occupies a fixed position against the TE, this spatial separation is usually straightforward to achieve under the microscope.
Taking too few cells risks insufficient DNA for reliable amplification and a higher "no result" / re-biopsy rate; taking too many cells (over-biopsy) is associated with reduced implantation potential in some studies. The 5–10 cell range represents a working balance between diagnostic yield and safety established through years of clinical practice and outcome data.
Multiple cohort studies and one randomized trial have examined whether TE biopsy itself harms embryo developmental potential. When performed by experienced embryologists on adequately expanded blastocysts, biopsy of 5–10 TE cells combined with vitrification has not been shown to meaningfully reduce implantation rates of euploid embryos compared to unbiopsied controls.
However, biopsy is an invasive procedure with a learning curve, and outcomes are highly technique- and center-dependent: excessive cell removal, biopsying poorly expanded blastocysts, or repeated laser exposure can measurably reduce viability. This is why PGT-A is generally recommended only through experienced IVF laboratories with dedicated micromanipulation expertise.
A biopsy of 5–10 cells contains only tens of picograms of genomic DNA — far too little to sequence directly. Whole genome amplification (WGA) multiplies this trace material roughly a million-fold, after which the amplified DNA is fragmented, barcoded, and pooled into a sequencing library while the biopsied embryo remains safely vitrified.
Two families of chemistry are used clinically:
• Multiple Displacement Amplification (MDA) — the phi29 DNA polymerase performs isothermal, highly processive strand-displacement synthesis from random primers, generating long amplification products with relatively even genome coverage, but with higher risk of allele dropout and chimeric artifacts
• PCR-based WGA (e.g., SurePlex, PicoPLEX, MALBAC-type chemistries) — genomic DNA is first fragmented and universally tailed, then amplified by PCR with common primers; more reproducible amplification bias that modern bioinformatics pipelines are calibrated to correct
Both methods introduce some amplification bias and “noise” across the genome, which is precisely why copy number calling algorithms rely on relative read-depth ratios between many genomic bins rather than absolute counts.
The amplified DNA is enzymatically or mechanically fragmented into short pieces (~200–500 bp), and platform-specific sequencing adapters plus a unique sample index (barcode) are ligated or tagged onto every fragment. This indexing step is critical — it allows dozens of embryo biopsies from many patients to be pooled and sequenced together on a single sequencing run, then computationally separated by their index after sequencing, dramatically reducing per-embryo cost and turnaround time.
Detecting whole-chromosome aneuploidy does not require deep, base-level sequencing coverage — it only requires counting how many reads land on each chromosome relative to the rest of the genome. This makes low-pass whole-genome sequencing (roughly 0.01–0.5× average coverage, i.e., only a small fraction of the genome is actually read per base) both statistically sufficient and highly cost-effective, typically requiring only a few million reads per embryo rather than the ~30× coverage needed for detecting single-nucleotide variants.
Low-pass NGS trades base-level resolution for chromosome-level counting power — millions of short reads are treated as a genome-wide "census," and it is the relative density of that census across each chromosome, not individual sequence variants, that reveals aneuploidy.
Once sequencing reads are generated, a bioinformatics pipeline aligns them to a reference genome, counts how many reads fall into thousands of genomic bins, corrects for known technical biases, and compares the result chromosome-by-chromosome against the expected diploid (two-copy) baseline across all 24 chromosome types — 1 through 22, plus X and Y.
The reference genome is divided into thousands of equal-sized bins. Sequencing reads are aligned, and the read count in each bin is corrected for GC content and mappability bias, then normalized against the genome-wide average. For a normal diploid (2-copy) region, the corrected read density clusters tightly around a baseline ratio; a chromosome present in 3 copies (trisomy) shows ~50% more reads than baseline, while a chromosome present in 1 copy (monosomy) shows ~50% fewer.
A segmentation algorithm (commonly a hidden Markov model or circular binary segmentation) then scans across each chromosome to call contiguous regions that deviate from baseline, producing a copy-number profile for the entire genome in one pass.
PGT-A (aneuploidy screening) is designed to detect:
• Whole-chromosome aneuploidy — an extra or missing copy of an entire chromosome (e.g., trisomy 21, monosomy X) • Large segmental imbalances — partial chromosome gains or losses generally larger than ~10 megabases
It is NOT designed to detect and will typically miss:
• Balanced structural rearrangements (balanced translocations, inversions) — no net gain or loss of DNA, so read depth is unchanged (this requires PGT-SR, structural rearrangement testing) • Single-gene / monogenic disorders — this requires PGT-M with disease-specific probes • Small copy number changes below the resolution floor, and point mutations or small indels
This distinction matters clinically: a "euploid" PGT-A result does not rule out monogenic disease or balanced rearrangements — it only addresses whole-chromosome and large segmental copy number status.
PGT-A infers the ploidy of the entire embryo from a proxy sample of only 5–10 trophectoderm cells. This assumes the biopsy is representative of the whole embryo — an assumption that mosaicism can violate.
If an embryo contains a mixture of chromosomally normal and abnormal cells (mosaicism, arising from post-zygotic mitotic errors), the biopsy read-depth ratio for the affected chromosome falls between the clean euploid and full-aneuploid values — for example, a chromosome present in an abnormal state in ~40% of biopsied cells produces an intermediate log2 ratio.
Modern platforms typically report a chromosome as "mosaic" when the estimated abnormal cell fraction falls roughly between 20% and 80%; below that floor, technical noise makes reliable discrimination from normal difficult, and above it, embryos are usually called fully aneuploid. Whether a given mosaic call reflects true biological mosaicism in the embryo versus WGA/sequencing artifact remains an active area of methodological debate.
Once every biopsied embryo in a cohort has a ploidy call, the clinical team ranks embryos for transfer: euploid embryos are prioritized for single embryo transfer, aneuploid embryos are typically deprioritized or not transferred, and mosaic embryos require individualized genetic counseling. Maternal age is the single strongest predictor of how many embryos in a cohort will actually be euploid.
Each embryo receives one of three broad categories:
• Euploid — normal copy number across all 24 chromosomes; highest priority for transfer, typically one embryo transferred at a time (elective single embryo transfer) • Aneuploid — a whole-chromosome or large segmental gain/loss; generally deprioritized, as most result in failed implantation, miscarriage, or (rarely, for viable trisomies like 13, 18, 21, or sex chromosome aneuploidies) an affected pregnancy — these embryos are usually not transferred • Mosaic — an intermediate call requiring individualized genetic counseling; some mosaic embryos, particularly those with lower abnormal cell percentages or involving certain chromosomes, have resulted in healthy live births when transferred after counseling, though outcomes are less predictable than euploid transfers
Embryos not selected for the current transfer remain safely vitrified in liquid nitrogen for potential future use.
The great majority of embryonic aneuploidy originates in the oocyte, during meiosis I, which begins before birth and can remain arrested for decades until ovulation. The leading explanation is age-related weakening of cohesin — the protein complex that holds paired chromosomes together until they are meant to separate. As cohesin degrades over the decades an oocyte spends arrested in meiosis, chromosomes are more likely to segregate incorrectly (nondisjunction) at ovulation, producing eggs with an extra or missing chromosome.
This is why euploidy rate among biopsied blastocysts falls steeply with maternal age — from roughly 65% around age 35 to roughly 20% by age 42 — while paternal (sperm-derived) aneuploidy contributes only a small, largely age-independent fraction of cases.
Because aneuploidy is overwhelmingly maternal-age-driven and meiotic in origin, PGT-A results are best interpreted as a probability shaped heavily by the age at which the eggs were retrieved — not as a fixed property of the IVF laboratory or protocol.
PGT-A is well supported for two specific outcomes: selecting a euploid embryo increases the implantation rate per embryo transferred, and it lowers the miscarriage rate per transfer, since a large share of early pregnancy loss is caused by aneuploidy. For patients of advanced maternal age or with recurrent pregnancy loss, this can mean fewer transfers and less time to a successful pregnancy.
However, several randomized controlled trials (e.g., the STAR trial) and registry analyses have found that in unselected or good-prognosis patients — younger patients with multiple good-quality blastocysts — PGT-A does not significantly increase the cumulative live birth rate per started IVF cycle. The reasoning is that many "aneuploid-labeled" or untested embryos in this population would still have produced live births if transferred anyway, while biopsy, vitrification, and the possibility of discarding a mosaic embryo that could have implanted normally introduce their own small risks and costs. This makes routine use of PGT-A in all IVF patients scientifically debated, even as it remains clearly useful in select populations (advanced maternal age, recurrent implantation failure, recurrent miscarriage).
| Product | Indication | Trial Design | Key Result |
|---|---|---|---|
| Euploid | All 24 chromosomes at expected copy number | ~65% of embryos at age 35; ~20% at age 42 | Priority for single embryo transfer |
| Aneuploid (whole-chromosome) | Full trisomy or monosomy of one or more chromosomes | Most common cause of implantation failure & miscarriage | Generally not selected for transfer |
| Segmental aneuploidy | Partial chromosome gain/loss, typically >10 Mb | May be inherited (unbalanced translocation) or de novo | Case-by-case; consider parental karyotype |
| Mosaic | Mixed euploid/aneuploid cell populations, ~20–80% abnormal | Post-zygotic mitotic error; biopsy may not represent whole embryo | Individualized genetic counseling required |