Too bad even that is debatable: the brain doesn't suddenly appear out of thin air, it slowly builds from simple neural connections, too simple to register as brain activity. I'm afraid there's not a single, clearly measurable event that can mark the difference between "clump of cells" and "potential person" but the fertilization of the egg. The other one is the fetus reaching enough homeostasis to survive outside of the womb without assistance for a few days, which would obviously be inhumane.