selfishness vs. instinct

Why is it that we {as women} are predisposed to have a "baby urge" when of a certain twirties age? Many women will tell you they don't. They're lying.
We all have them; some are just better at keeping The Push at bay.
Sickening is the overwhelming desire I have for an infant when I see one, typically with its cute, non-working mother strolling it in and out of the shade Uptown. (Though seeing one with a Dad is just pure torture. That's fodder for some other post.) It's an instinctual longing and pain of heart.
Controlling the follow-up thoughts are easy - no money, no time, no patience, no husband, etc. The list is endless. Unfortunately, though, it's those initial "I want one. I should have one by now. What if I'm too old and I never have one" that make me ill.
And of course, those are typically followed immediately by "What if I'm never married? I don't want to die alone. How is it that I've ended up in this position? Why can't I make more money?" ... etc.
The upside is that their hang time is short - all of these fleeting feelings whiz past in less than a second. So I guess as a human race we've done a decent job of overshadowing our instincts with selfishness.
Because I don't see the average age of marriage getting any younger.