Genetic and Epigenetic Mechanisms Underlying Phenotypic Discordance in Monochorionic Monozygotic Twins: A Systematic Review

Background: Monochorionic twin pregnancies provide a unique model to investigate fetal phenotypic discordance, because both fetuses share a single placenta and interconnected vascular circulation. Although most monochorionic twins are monozygotic, clinically relevant differences may arise through genetic, epigenetic, placental, and stochastic developmental mechanisms. Methods: This systematic review was conducted according to PRISMA 2020 and registered in PROSPERO (CRD420261432361). PubMed/MEDLINE, Embase, and Scopus were searched from inception to June 2026. Eligible studies included monochorionic monozygotic twin pairs with discordant congenital, developmental, or syndromic phenotypes, confirmed or clearly inferable monochorionicity, and at least one genomic, cytogenetic, or epigenetic investigation; studies describing confirmed monochorionic dizygotic twinning were excluded. Findings were synthesized qualitatively. Results: The search identified 1357 records. After duplicate removal and screening, 48 studies fulfilled the eligibility criteria, comprising 441 monozygotic twin pairs; 37 were single-pair case reports, whereas one large retrospective cohort study alone contributed 193 pairs (44% of the entire pooled sample). Reported phenotypes included congenital heart disease, chromosomal abnormalities, disorders of sex development, imprinting disorders, neurodevelopmental disease, endocrine disorders, renal anomalies, skeletal disorders, and multisystem malformations. Molecular methods included karyotyping, FISH, chromosomal microarray, array-CGH, CNV analysis, WES, WGS, targeted sequencing, and methylation profiling. Proposed mechanisms included postzygotic chromosomal errors, somatic mutations, tissue-specific mosaicism, discordant or shared CNVs, differential methylation, imprinting defects, variable expressivity, blood chimerism, unequal placental sharing, TTTS, TAPS, sFGR, and uteroplacental insufficiency. Conclusions: Phenotypic discordance in monochorionic twins is rarely explained by a single mechanism. Available evidence supports a multifactorial model in which postzygotic genetic events, epigenetic regulation, placental vascular factors, and stochastic developmental processes interact.