Magnetic resonance spectroscopy (MRS) can measure the concentration of a range of metabolites to understand pathophysiological and normal development in the fetus, as well as provide diagnostic and prognostic markers . However, MRS applications in the fetus are technically very challenging due to the inherently low SNR (fetal size, distance from the coil, the use of 1.5T scans) and signal disruption caused by unrestricted fetal motion during data collection. Given the limited available SNR, outlier rejection to remove motion corrupted averages and improve spectral quality is particularly important. Spectral phase shifts are associated with motion, both during signal acquisition and between MRS repeats . We propose a threshold-free method for optimal outlier rejection of fetal MRS, “Ranked residual-water Phase Shift”: the phase shift in the residual water after water suppression is used as a metric linked to potential motion in individual transients/repeats.