Fetus-in-Fetu: A PRISMA-Informed Narrative Review With an Illustrative Prenatal Case

Area of Science:

• Developmental Biology
• Medical Genetics
• Congenital Anomalies

Background:

• Fetus-in-fetu (FIF) is a rare congenital anomaly resulting from aberrant monozygotic twinning.
• FIF is often misdiagnosed as a teratoma, especially when detected later in life or at unusual locations.

Purpose of the Study:

• To provide a comprehensive review of human literature on FIF.
• To integrate embryological, diagnostic, genetic, and surgical aspects of FIF.
• To highlight diagnostic challenges and management strategies.

Main Methods:

• A PRISMA-informed narrative synthesis of 45 identified publications.
• Selection of 25 core studies for in-depth analysis.
• Inclusion of a novel case report with omphalocele and cardiac anomalies.

Main Results:

• FIF is most commonly diagnosed in infancy and is typically benign after complete surgical excision.
• Key diagnostic features include organized axial structures, symmetry, and shared monozygotic genetics, differentiating it from teratomas.
• Malignant transformation is rare, but surveillance is needed for incomplete resections or immature tissues.

Conclusions:

• Improved prenatal imaging aids early FIF recognition, though developmental mechanisms require further understanding.
• Distinguishing FIF from teratomas relies on specific structural and genetic markers.
• Future research should focus on embryonic asymmetry, diagnostic accuracy, and evidence-based follow-up.