The fetus–in–fetu is a very rare abnormal embryogenesis of twinning where a malformed prasitic foetus is found in the body of its twin. Case Study: This report presents a case of fetus–in–fetu diagnosed prenatally using Doppler ultrasonography. Discussion: “Fetus–in–fetu” is estimated to occur in 1 in 5,00,000 live births. This rare congenital anomaly occurs due to aberration in the diamniotic monozygotic twinning with unequal division of inner cell mass of blastocyst leading to inclusion of small cell mass inside the developing host foetus. Only about 100 cases are reported worldwide. Various etiologies are discussed in the present study. Conclusion: Fetus–In–Fetu is a rare entity that typically presents as an abdominal mass in infancy or early childhood. With recent advances in prenatal sonography, the diagnosis of Fetus–In–Fetu, as shown in this case, can be made more accurately in the prenatal period.