Numerosity discrimination has been demonstrated in newborns, but not in fetuses. Fetal magnetoencephalography allows non-invasive investigation of neural responses in neonates and fetuses. During an oddball paradigm with auditory sequences differing in numerosity, evoked responses were recorded and mismatch responses were quantified as an indicator for auditory discrimination. Thirty pregnant women with healthy fetuses (last trimester) and 30 healthy term neonates participated. Fourteen adults were included as a control group. Based on measurements eligible for analysis, all adults, all neonates, and 74% of fetuses showed numerical mismatch responses. Numerosity discrimination appears to exist in the last trimester of pregnancy.