The problem with this logic is that you would never hire a woman, for fear that they could get pregnant and become "unreliable" for 9 months, and completely unavailable for the duration of their maternity leave (while having to keep their job position unfilled).

It's far better to apply some humanity to the equation and just give everyone those extra 12 days off (and (mat|pat)ernity leave), so you're not tempted to discriminate off the "productivity loss".