The twin anemia-polycythemia sequence (TAPS) and acute (peripartum) intertwin transfusion (AITT) are rare forms of transfusions between twins in monochorionic (MC) multiple pregnancy. These types are met less often than twin-to-twin transfusion syndrome (TTTS), the prevalence is 2–3% for TAPS after laser ablation and 3–6% spontaneously and 1.5–2.5% for AITT in comparison with 9–15% for TTTS in MC twins. The understanding of natural history and staging of the development of anemia-polycythemia helps explain some cases of unexpectedly unfavorable perinatal outcomes in newborn twins with the considerable hematological discordance without presence of remarkable prenatal signs of twin-to-twin transfusion. As opposed to the conventional wisdom, TAPS may be associated with the significant amniotic fluid discordance besides typical US signs such as discrepancy of twins’ peak systolic velocity in middle cerebral artery (MCA PSV). The TAPS may not have the progredient antenatal pattern. In discordant values of PSV MCA, the extended monitoring of the fetal cardiovascular profile is required to estimate the degree of perinatal risk as well as select the optimal time for delivery. Postnatally, the differential diagnosis of AITT and TAPS is based on the logics of retrospective of fetal MCA PSV values. The AITT diagnosis may be assumed for the MC twins with normal antenatal MCA Doppler velocities and episodes of bradycardia of fetuses during the delivery. At the same time, TAPS that is being developed just before the delivery can hardly be certainly ruled out. This article focuses on the review of current views as for perinatal diagnosis of rare forms of intertwin transfusion as well as on particularities of some clinical cases with unexpected association of symptoms. It is shown that some unusual symptoms of TTTS and TAPS as well as their mixed forms influence the US images and complicate the classification, staging, and prognosis of the abnormalities.