Much like womanhood, the only clear defining element is that you stand up proud and declare to the world “I am a mother”.
Umm, what now? If a childless woman declared herself to be a mother, most people would wonder if she was joking, or delusional, or expressing a wish, or using the word in a metaphorical sense (e.g. caring for pets). They certainly wouldn't accept that she was literally a mother just because she said so.
Or maybe they would, maybe that's how the world works these days.