About three months after I’d given birth, I was invited to a recently-befriended mum’s house for a gathering of new parents and babies. As my bleary brain tried desperately to remember each adult’s name, and at least the gender of their corresponding tot, I suddenly realised we were one cooing, spewing infant short of a full set. How odd. Later, as I happened on the lady I’d identified as childless refilling her mug in the kitchen, I inquired about the “little one”, wholly prepared to praise her willingness to leave him or her with someone else at such a young...