Measurement of blood flow in the fetal heart and the great vessels is challenging due to fetal motion and its small size. Previously, we demonstrated use of k-t SENSE real-time 2D imaging combined with slice-to-volume registration to reconstruct 4D velocity cine volumes. This required 50% of the examination to be spent acquiring training data. In this work we combine sliding window reconstruction of the under-sampled target data with some prior knowledge to dispense with training data altogether. We reconstruct 4D blood flow volumes in 5 fetal hearts using both methods and show that they are broadly equivalent.