Recently several dear friends have written to tell me of their number twos on the way. Each note registers as a tiny shock to my system. I saw these children coming; I expected them in the natural course of things. But I always thought I would respond with my own exclamation, my little bit of thrilling news. Not long ago, these joyous announcements would have laid me out flat with envy; I craved another baby and couldn’t imagine my life would be complete otherwise. The dream of two (or even more) children, of spending years at a time with those small, warm bodies and soft, shiny faces, wouldn...