3.10 Assisted Reproductive Technologies & PGT
Key Takeaways
- IVF creates embryos in vitro after ovarian stimulation and egg retrieval; ICSI injects a single sperm into each oocyte and is used for male-factor infertility, prior fertilization failure, and many PGT cycles
- PGT-M tests embryos for a known familial monogenic pathogenic variant (or related haplotype); PGT-A screens for aneuploidy; PGT-SR evaluates unbalanced products of a parental structural rearrangement
- Most modern PGT uses trophectoderm biopsy at the blastocyst stage; mosaicism in biopsied cells may not represent the inner cell mass that becomes the fetus
- PGT-A does not guarantee a euploid pregnancy, eliminate miscarriage, or replace diagnostic prenatal testing when indicated; counseling must include false-negative/false-positive and mosaic-result pathways
- Genetic counselors frame ART/PGT as one reproductive option among several, clarify what the test can and cannot answer, and coordinate with reproductive endocrinology and the PGT laboratory on probe design and report interpretation
3.10 Assisted Reproductive Technologies & PGT
Quick Answer: IVF fertilizes eggs outside the body; ICSI injects one sperm into each oocyte. PGT-M targets a known monogenic familial variant; PGT-A screens embryos for aneuploidy; PGT-SR looks for unbalanced products of a parental rearrangement. Biopsy is usually trophectoderm at blastocyst stage—mosaicism can misrepresent the fetus—so PGT informs transfer decisions but does not erase residual risk or replace indicated prenatal diagnosis.
Domain 3C items ask whether you can match the reproductive technology to the genetic question, explain limitations before a cycle starts, and counsel mosaic or inconclusive PGT results without overselling certainty. You are not expected to run an IVF lab; you are expected to translate ART/PGT into accurate risk language and next steps.
IVF and ICSI: What Genetic Counselors Need
In vitro fertilization (IVF) typically includes controlled ovarian stimulation, monitoring, egg retrieval, fertilization, embryo culture (often to blastocyst, day 5–7), and embryo transfer (fresh or frozen). Cryopreservation allows deferred transfer and is standard when PGT results are pending.
Intracytoplasmic sperm injection (ICSI) places a single spermatozoon into the oocyte cytoplasm. Common indications: severe male-factor infertility, prior failed conventional IVF fertilization, use of surgically retrieved sperm, and many cycles when PGT is planned (to reduce contamination risk from excess sperm DNA on the zona).
| Modality | Core idea | GC counseling angle |
|---|---|---|
| IVF (conventional insemination) | Sperm and eggs co-incubated | Discuss cycle burden, multiple-gestation policy if multiple embryos transferred, and that genetics of the embryo still depends on parental gametes |
| ICSI | One sperm injected per egg | Male-factor context; slight differences in fertilization mechanics—not a genetic “fix” for hereditary disease |
| Embryo cryopreservation / FET | Freeze-all or freeze surplus; later frozen embryo transfer | Time for PGT results; counseling continues across cycles |
| Donor gametes / gestational carrier | Third-party reproduction | Separate informed-consent and psychosocial layers (see 3.11) |
Exam trap: Equating IVF with PGT. Many IVF cycles have no genetic testing. PGT is an add-on indication and laboratory process, not automatic with every ART cycle.
PGT Categories: Match Test to Question
Preimplantation genetic testing (PGT) analyzes a biopsy from an embryo before transfer. Naming follows indication:
| Type | Clinical question | Typical candidates |
|---|---|---|
| PGT-M (monogenic) | Does this embryo inherit the familial pathogenic variant (or linked haplotype) causing a known single-gene disorder? | Couples where both are carriers of an autosomal recessive condition; autosomal dominant affected parent; X-linked scenarios; some mitochondrial setups with specialized approaches |
| PGT-A (aneuploidy) | Is the embryo’s chromosome copy number consistent with euploidy (within assay limits)? | Often used to select embryos for transfer in infertility/advanced maternal age settings; not a substitute for diagnostic testing of a pregnancy |
| PGT-SR (structural rearrangement) | Is this embryo balanced/normal vs unbalanced for a known parental translocation/inversion? | Balanced translocation or inversion carriers with history of unbalanced offspring, miscarriage, or infertility |
PGT-M essentials. The lab needs a known familial variant (or validated linkage/haplotype markers) and usually DNA from the affected individual and/or carrier parents to design a robust assay. Direct mutation testing, linkage, or combined strategies reduce allele dropout risk. Counsel that PGT-M reduces the chance of transferring an affected embryo for that specific indication; it does not screen the entire genome for unrelated conditions unless additional testing is explicitly ordered.
PGT-A essentials. Platforms (often NGS-based copy-number analysis on trophectoderm) report euploid, aneuploid, or mosaic calls. PGT-A aims to improve selection efficiency—it does not create euploid embryos, does not eliminate all miscarriage risk, and has uncertain benefit in every age/indication group. Societies have debated routine use; exam vignettes reward limitation-aware counseling over marketing claims.
PGT-SR essentials. Unbalanced segregation products are the target. A “normal/balanced” PGT-SR result may not distinguish a truly normal karyotype from a balanced rearrangement identical to the parent’s unless the assay is designed to do so—clarify with the lab what “transferable” means for that family’s rearrangement.
Trophectoderm Biopsy and Mosaicism
Modern practice favors blastocyst trophectoderm biopsy (extraembryonic lineage) rather than day-3 blastomere biopsy. The inner cell mass (ICM) forms the fetus; trophectoderm contributes primarily to placenta. Therefore:
- A mosaic trophectoderm result may reflect confined placental-type mosaicism, true embryonic mosaicism, or technical artifact.
- An apparently euploid biopsy can still miss low-level mosaicism or later mitotic errors.
- An aneuploid call in trophectoderm is often non-transfer in clinic policy, but mosaic-result pathways vary—some centers transfer selected mosaics with intensive counseling and offer prenatal diagnostic confirmation.
| Finding | Counseling frame |
|---|---|
| Euploid PGT-A | Lower aneuploidy likelihood for assayed chromosomes/regions—not zero residual risk; prenatal screening/diagnostic options still exist |
| Aneuploid PGT-A | Typically not transferred; discuss discard vs research donation per clinic ethics/consent |
| Mosaic PGT-A | Uncertainty about fetal representation; review chromosome involved, mosaic level if reported, clinic policy, and plan for CVS/amnio if pregnancy occurs |
| No result / uninformative | Technical failure, amplification issues, or poor biopsy—may need re-biopsy policy or transfer without PGT information |
Worked scenario. A couple both carry the same CFTR pathogenic variants pursues IVF with PGT-M. Embryos classified as non-affected for CF may still be aneuploid if PGT-A was not performed. Conversely, a euploid PGT-A embryo from a different couple can still inherit a monogenic disease that was never tested. Indication drives test menu.
Limitations and Confirmatory Testing
Board-relevant limitations to state before stimulation:
- Allele dropout / ADO (especially older single-cell methods; still conceptually relevant): false unaffected call for PGT-M if the mutant allele fails to amplify—mitigated by linkage markers and modern protocols.
- Mosaicism and sampling error as above.
- Segmental aneuploidy and intermediate copy-number calls may be platform-dependent.
- Mitochondrial DNA disorders and some repeat-expansion disorders may need specialized approaches; not all conditions are equally PGT-amenable.
- Cost, emotional burden, time to pregnancy, and possible cycle cancellation if few or no transferable embryos.
If pregnancy is achieved after PGT, counselors often discuss whether prenatal diagnostic testing (CVS or amniocentesis with targeted molecular and/or chromosomal analysis) is desired for confirmation—especially after mosaic transfer, complex PGT-SR, or high parental anxiety. NIPT remains a screening tool and does not confirm PGT-M results for a monogenic variant.
GC Role Across the ART Pathway
- Preconception: Confirm molecular diagnosis in the family, inheritance, residual risks without ART, and alternatives (see 3.11).
- Lab coordination: Variant documentation, sample requirements for probe workup, turnaround before a planned cycle.
- Results session: Translate embryo reports into transfer decisions with reproductive endocrinology; avoid directive pressure toward or against PGT.
- Pregnancy: Offer confirmatory testing pathways and standard prenatal care coordination.
Master the PGT-M vs PGT-A vs PGT-SR distinction and the trophectoderm mosaicism counseling frame—these drive most Domain 3C ART vignettes.
A couple in which one partner carries a balanced reciprocal translocation seeks embryo testing specifically to avoid transferring unbalanced embryos. Which PGT category best matches this indication?
Why does trophectoderm mosaicism complicate counseling after blastocyst PGT-A?
Both members of a couple are carriers of the same autosomal recessive pathogenic variants and pursue IVF with testing so unaffected embryos can be prioritized for transfer. Which statement is most accurate?
A patient asks whether a euploid PGT-A result means prenatal chromosomal testing is never needed if she becomes pregnant. The most appropriate counseling response is: