An utterance, voice, thought or dream that comes as one ages, that, even though logically one does not want a baby, urges one to be fruitful and multiply.
Aging woman: I had a dream last night that I was nursing a baby, and now I can't stop thinking about it. I want a baby so bad now.
Friend: Don't worry... that's just the bioclock speaking and doesn't mean anything.
Aging woman: (thoughtfully) I'm ovulating today...
Friend: You know you don't want that. Do not heed when the voice of the bioclock speaks!