Our objective was to explain the clinical presentations of sustained arteriovenous anastomotic transfusion of blood after incomplete laser therapy in twin-to-twin transfusion syndrome (TTTS). We extended our mathematical model of TTTS by adding the dynamics of hematocrit, and simulated incomplete laser therapy, first, by leaving one patent opposite arteriovenous anastomosis from the recipient to the donor and, second, by leaving one patent arteriovenous anastomosis from the donor to the recipient. In both simulations we reproduced the clinical observation of severe hematocrit discordance preceding delayed amniotic fluid imbalance. In conclusion, incomplete laser therapy may cause a severe circulatory imbalance between the twins which presents predominantly as discordant hematocrits rather than discordant amniotic fluid volumes as in primary TTTS. These results imply that the anemia-polycythemia sequence is a sensitive mechanism to identify transfusion reversal after complicated laser therapy, confirming the suggested role of middle cerebral artery peak systolic velocity Doppler measurements as a useful method of follow-up.