The basic neuronal substrate required to transmit somatosensory information develops by mid-gestation (18 to 25 weeks), however, the functional capacity of the neural circuitry is limited by the immaturity of the system. Thus, 18 to 25 weeks is considered the earliest stage at which the lower boundary of sentience could be placed. At this stage of development, however, there is little evidence for the central processing of somatosensory information. Before 30 weeks gestational age, EEG activity is extremely limited and somatosensory evoked potentials are immature, lacking components which correlate with information processing within the cerebral cortex. Thus, 30 weeks is considered a more plausible stage of fetal development at which the lower boundary for sentience could be placed.

Tell them this. Fetuses have no more ability to percieve or feel than plants do. If they still are obtuse about it, ask them if they consider male ejaculation or PMSing to be abortion (as sperm/eggs are the begginning stages of life).