Preimplantation genetic testing for structural rearrangements — screening embryos of balanced translocation carriers
A balanced structural rearrangement carrier has the normal total complement of genetic material, just rearranged in position. This causes no phenotypic effect in the carrier — but it fundamentally destabilizes chromosome pairing and segregation during their own meiosis, putting every pregnancy at elevated risk of an unbalanced conception.
A structural rearrangement is balanced when no genetic material is gained or lost, only repositioned. Three forms are relevant to PGT-SR:
Reciprocal translocation: two non-homologous chromosomes exchange terminal segments after double-strand breaks, e.g. t(11;22)(q23;q11) — one of the most common recurrent human translocations. The carrier ends up with two "derivative" chromosomes (der11, der22) alongside one normal 11 and one normal 22.
Robertsonian translocation: two acrocentric chromosomes (13, 14, 15, 21, or 22 — the ones with the short arm reduced to non-essential ribosomal DNA repeats) fuse at or near the centromere, with loss of the two short arms. This reduces the chromosome count to 45, but no unique gene-coding material is lost, so the carrier is clinically balanced.
Inversion: a single chromosome breaks in two places and the intervening segment reinserts in reverse orientation. Paracentric inversions do not include the centromere; pericentric inversions do — the two forms produce different meiotic pairing loops and different unbalanced-gamete mechanisms.
In all three cases, karyotyping (G-banding) or chromosomal microarray on a peripheral blood sample identifies the carrier — usually only after a couple presents with recurrent miscarriage, infertility, or a previously affected child.
Balanced translocation carriers are found in roughly 1 in 500 people in the general population, but in 2–5% of couples with recurrent pregnancy loss — a roughly 10-fold enrichment that makes karyotyping a standard recurrent-loss work-up.
Conventional G-banded karyotyping resolves rearrangements down to roughly 5–10 Mb — enough to detect the translocation, but not precise enough to design a targeted NGS assay. Once a carrier is identified, breakpoint mapping is refined using:
• FISH (fluorescence in situ hybridization) with region-specific probes flanking the suspected breakpoint • Chromosomal microarray (array CGH / SNP array) to narrow the breakpoint interval • Mate-pair or long-read sequencing in specialized cases, to resolve the breakpoint to base-pair resolution
This breakpoint map becomes the reference against which every embryo's trophectoderm biopsy is later compared. Without it, a genome-wide NGS scan could still catch large imbalances, but a breakpoint-informed design dramatically improves sensitivity for the specific small duplication/deletion segments this particular couple is at risk of producing.
A balanced carrier's own somatic cells function normally because every gene is present in the correct dosage — position does not matter for most genes, only copy number. The risk appears specifically during meiosis, when the rearranged chromosomes must pair with their normal homologs.
Because a reciprocal translocation involves two different chromosomes, the four relevant chromosomes (2 normal + 2 derivative) must associate into a four-armed pairing structure (a quadrivalent) to allow homologous regions to synapse. Robertsonian carriers must pair a trivalent of three chromosomes. Inversion carriers form a pairing loop along the inverted segment.
Whichever segregation pattern subsequently resolves this structure — covered in Stage 2 — determines whether each resulting gamete carries a full, balanced set of chromosome material or a duplication/deletion. This is why balanced carriers experience recurrent miscarriage, infertility, or occasionally a liveborn child with a serious unbalanced karyotype, despite being completely healthy themselves.
The quadrivalent (reciprocal) or trivalent (Robertsonian) pairing structure formed at meiosis I can resolve along several different geometric axes. Each segregation pattern sends a different combination of chromosomes into each daughter cell — some patterns reconstruct a balanced set, most do not.
At pachytene, the two normal chromosomes and the two derivative chromosomes of a reciprocal translocation carrier associate into a cross-shaped, four-armed quadrivalent so that homologous segments can synapse along their full length. At anaphase I, this structure resolves into one of several segregation patterns:
• Alternate segregation: chromosomes positioned diagonally opposite each other in the cross segregate together. This sends either both normal chromosomes to one pole (a fully normal gamete) or both derivative chromosomes to the other pole (a balanced-carrier gamete). Both outcomes are chromosomally balanced.
• Adjacent-1 segregation: homologous centromeres separate from each other, but non-homologous (adjacent) chromosomes travel together — pairing one normal chromosome with one derivative that share no homology. This produces gametes with a duplication of one segment and a deletion of another — unbalanced.
• Adjacent-2 segregation: homologous centromeres travel together (violating normal disjunction), producing a different unbalanced duplication/deletion combination. This pattern is mechanistically rarer than adjacent-1 because it requires centromeres to miss-segregate.
• 3:1 (tertiary) segregation: the quadrivalent splits unevenly, three chromosomes to one pole and one to the other, producing gametes that are effectively trisomic or monosomic for part of the rearranged chromosomes — the most severely unbalanced outcome.
Across all segregation patterns combined, only about 50–70% of gametes produced by a reciprocal translocation carrier are chromosomally balanced (normal or balanced-carrier) — the remainder are unbalanced and, if fertilized, are the major driver of the carrier's elevated miscarriage risk.
Robertsonian carriers pair only three chromosomes at meiosis I: the fused derivative and the two remaining normal acrocentric homologs, forming a trivalent rather than a quadrivalent. Segregation is simpler but still risky:
• Alternate segregation: the derivative segregates from both normal acrocentrics together, producing either a normal gamete (one normal copy of each acrocentric) or a balanced-carrier gamete (the fused derivative alone) — both balanced.
• Adjacent segregation: the derivative segregates together with one normal homolog, producing a gamete that is disomic for one acrocentric and nullisomic for the other — unbalanced, resulting in trisomy or monosomy of that chromosome after fertilization.
Because Robertsonian translocations most often involve chromosomes 13, 14, 15, 21 and 22, the clinically important unbalanced outcomes mirror the free trisomies of those chromosomes — including translocation Down syndrome (trisomy 21) when a rob(14;21) or rob(21;22) carrier undergoes adjacent segregation. Homologous Robertsonian translocations, such as rob(13;13) or rob(21;21), are a special case: alternate segregation is impossible, so essentially every viable conception is unbalanced.
Segregation is not a simple coin flip. Multiple factors skew the ratio of balanced to unbalanced gametes for a given carrier:
• Chromosome size and breakpoint position: translocations involving small distal segments are more likely to favor alternate segregation; larger exchanged segments increase the physical bulk of the quadrivalent and can favor adjacent segregation • Sex of the carrier: female carriers generally show somewhat higher rates of unbalanced gametes than male carriers for the same rearrangement, likely reflecting differences in meiotic checkpoint stringency between oogenesis and spermatogenesis • Interchromosomal effect: the presence of a quadrivalent/trivalent can modestly increase nondisjunction rates even among chromosomes not involved in the rearrangement
Because this bias is rearrangement-specific, empirical segregation data from prior pregnancies or sperm/embryo studies for a given breakpoint pair is often more informative than population averages when counseling a specific couple.
| Product | Indication | Trial Design | Key Result |
|---|---|---|---|
| Alternate | Diagonal chromosomes co-segregate | Both normals to one pole, both derivatives to the other | Balanced — normal or balanced-carrier gamete |
| Adjacent-1 | Non-homologous centromeres co-segregate | One normal + one non-homologous derivative per pole | Unbalanced — duplication / deletion |
| Adjacent-2 | Homologous centromeres co-segregate | Requires centromere mis-disjunction; mechanistically rarer | Unbalanced — different duplication / deletion |
| 3:1 Tertiary | Uneven 3-vs-1 chromosome split | Quadrivalent resolves asymmetrically | Unbalanced — partial trisomy / monosomy |
PGT-SR does not test gametes directly — it tests the resulting blastocyst. A small, expendable sample of trophectoderm cells (the precursor of the placenta, not the fetus itself) is removed on day 5–6 of development and profiled by next-generation sequencing, referenced against the couple's previously mapped breakpoints.
By the blastocyst stage, the embryo has differentiated into two lineages: the inner cell mass (ICM), which will become the fetus, and the trophectoderm (TE), an outer ring of cells that will form the placenta and extraembryonic tissue. Because TE cells share the embryo's genome but are not fated to become fetal tissue, removing 5–8 of the ~100–200 TE cells for analysis carries minimal risk to ongoing development — unlike day-3 cleavage-stage biopsy, which removes a much larger fraction of a much smaller embryo.
TE biopsy also provides more DNA and captures a population-level snapshot rather than a single cell, buffering against amplification dropout and reducing the impact of confined mosaicism (where the TE and ICM genetically diverge) compared to single-cell approaches.
A trophectoderm biopsy yields only picograms of genomic DNA — far below the input requirement of standard NGS library preparation. Whole-genome amplification (WGA), typically via multiple displacement amplification or a PCR-based method, uniformly amplifies this trace material more than a thousand-fold before sequencing.
The amplified DNA is then converted into a sequencing library and run on a low-pass, genome-wide next-generation sequencing platform — the same underlying chemistry used for PGT-A (aneuploidy screening), typically 1–5 million reads per embryo, far below deep clinical sequencing but sufficient for binned copy-number analysis across ~1 Mb windows.
A generic low-pass NGS scan, as used for standard PGT-A, can detect whole-chromosome aneuploidy and large segmental imbalances, but a translocation-derived duplication/deletion can be small — sometimes just a few megabases at the exchanged segment tips. Standard PGT-A bioinformatic pipelines are not tuned to reliably call such focal imbalances.
PGT-SR addresses this by building a custom analysis specifically informed by the couple's mapped breakpoints (from Stage 1): read-depth bins are placed with extra density flanking the known breakpoint coordinates on the derivative chromosomes, and the expected duplication/deletion signature for each possible unbalanced segregation product (Stage 2) is modeled in advance. This raises sensitivity for exactly the small, recurrent imbalances this specific couple is at risk of transmitting, while still retaining genome-wide coverage to catch de novo aneuploidy unrelated to the known rearrangement.
Whole-genome amplification combined with breakpoint-informed NGS can now resolve structural-rearrangement-related copy-number imbalances down to a resolution of several megabases — small enough to catch most clinically relevant unbalanced translocation products that would be invisible to a standard karyotype.
Read-depth data from each embryo is converted into a genome-wide log2 copy-number ratio profile. Segments matching the parental breakpoint coordinates are compared against the expected signatures of each segregation outcome from Stage 2, and every embryo is sorted into one of three clinically actionable categories.
For a euploid, structurally normal embryo, read depth across every chromosome bin clusters tightly around a log2 ratio of 0 — two copies of every segment, no more, no less. For a balanced-carrier embryo (one that inherited the derivative chromosomes via alternate or alternate-equivalent segregation), the profile is also flat at log2 ≈ 0, because although the chromosomes are rearranged in position, no material is gained or lost — copy-number analysis alone cannot distinguish a balanced carrier embryo from a fully normal one.
For an unbalanced embryo, the bins spanning the duplicated segment shift upward (log2 ≈ +0.5, three copies) and the bins spanning the deleted segment shift downward (log2 ≈ −1.0, one copy), with a sharp transition exactly at the mapped breakpoint coordinate — the signature that breakpoint-informed analysis is specifically tuned to detect with high confidence even when the imbalanced segment is only a few megabases long.
Because copy-number sequencing cannot distinguish a balanced-carrier embryo from a fully normal embryo — both show a flat log2 profile — some PGT-SR programs offer optional targeted assays (e.g. long-read or junction-spanning PCR across the breakpoint) to identify which "balanced" embryos are carriers versus fully normal, mainly for family-planning counseling rather than transfer eligibility.
Clinically, this distinction rarely changes management: both normal and balanced-carrier embryos are chromosomally euploid and safe to transfer. A balanced-carrier child will, like their parent, be phenotypically unaffected but may face the same reproductive planning question in adulthood — information that can be relayed for future counseling without altering which embryos are prioritized now.
Because the same low-pass genome-wide NGS data used for breakpoint-informed analysis also covers every other chromosome, PGT-SR panels concurrently screen for whole-chromosome aneuploidy unrelated to the familial rearrangement — effectively combining PGT-SR with PGT-A in a single test.
This matters because embryo aneuploidy risk (which rises steeply with maternal age) is independent of translocation-derived imbalance risk. An embryo can simultaneously be balanced for the familial rearrangement and aneuploid for an unrelated chromosome (e.g., trisomy 16), or vice versa. Only embryos that are clear on both axes — balanced/normal at the rearrangement and euploid genome-wide — are prioritized for transfer.
Because balanced-carrier and fully normal embryos are bioinformatically indistinguishable by copy number alone, both are equally eligible for transfer — clinicians do not need to (and generally cannot, without extra assays) tell them apart to select a chromosomally safe embryo.
The final step is deceptively simple: transfer only embryos classified as normal or balanced. This single filtering decision converts a pregnancy at 50–70% miscarriage risk into one with a risk profile close to that of the general population — the entire clinical rationale for offering PGT-SR to known carriers.
Without testing, a known translocation or inversion carrier conceiving naturally faces a substantially elevated risk of miscarriage — commonly cited at 50–70%, reflecting the fraction of conceptions arising from unbalanced gametes across all segregation patterns (Stage 2). Some unbalanced combinations are so severely imbalanced that they are non-viable and end in early loss; a smaller number survive to result in a liveborn child with an unbalanced karyotype and associated congenital anomalies.
PGT-SR intercepts this at the embryo stage, before transfer, rather than at the prenatal-diagnosis stage after implantation. By classifying every biopsied embryo (Stage 4) and transferring only those with a normal or balanced complement, the unbalanced fraction is removed from the reproductive pool entirely — the couple's underlying meiotic segregation bias no longer determines pregnancy outcome, because it was resolved before transfer.
Selecting only balanced or normal embryos for transfer reduces a known carrier's miscarriage risk from roughly 50–70% under natural conception down to approximately 10–15% — essentially the same background miscarriage rate seen in the general, non-carrier population.
Depending on the segregation pattern and chromosomes involved, roughly 30–50% of biopsied embryos from a carrier cycle are classified as unbalanced and are not prioritized for transfer. For carriers with especially unfavorable segregation biases (e.g., homologous Robertsonian translocations such as rob(13;13), where alternate segregation cannot occur), this fraction can approach 100%, and cycles may need to be repeated or donor gametes considered.
Unbalanced embryos are not discarded arbitrarily — programs follow the same ethical and regulatory frameworks (informed consent, storage/disposition preferences) that govern all non-transferred embryos in IVF, and couples are counseled on the specific unbalanced outcome pattern observed, which can refine expectations for future cycles.
PGT-SR substantially reduces but does not eliminate risk. Residual sources of error include:
• Resolution limits: imbalances smaller than the assay's effective resolution (typically several Mb) may be missed • Mosaicism: if the trophectoderm biopsy does not perfectly represent the inner cell mass, a call can be discordant with the fetal karyotype • De novo events: new structural or numerical abnormalities arising independently of the familial rearrangement are still possible, though concurrent genome-wide screening catches most whole-chromosome cases • No test replaces prenatal confirmation: professional guidelines recommend confirmatory prenatal diagnosis (chorionic villus sampling or amniocentesis) even after a favorable PGT-SR result, given these residual limitations
With these caveats, PGT-SR remains the most effective intervention available for known structural rearrangement carriers seeking to reduce miscarriage risk and the chance of an unbalanced liveborn outcome, and is now considered standard of care for carriers pursuing IVF.