Couples who decided not to have babies, why?

I'm seeing people, who are now in their mid thirties not planning for a baby. My own boss and colleagues don't have babies. I wonder why that is so. Is it that they think (especially women) that having babies will hamper their career growth?

I myself am leaning towards not having a baby in the future. Wanted your take on why you'd do that.

Edit: People who had babies, what's your take?