one doesn't track who is going to conceive a child beforehand, one uses carrots and sticks as needed once the child is born and allows the rest of the population to modify their behaviour accordingly.
(ironically in the context of this response, a system used with great success by china to enforce their one-child policy at the end of the last century without anywhere near the level of tracking which is commonplace in most countries today.)
i was referring to voters in my own country (india), but i guess point is universally applicable to all democracies?