I'm no genius, but I'm going to assume that most people are not best friends with their bosses. Sure, they might like each other, but they're not going to be included in the wedding party. (They might be lucky to get an invite to the reception.)
Yet, when a woman finds out she's going to have a baby, the shortlist of people to tell inevitably includes the boss. Suddenly someone you don't want anywhere near your bedroom is the first to hear about your pregnancy.