I cannot recall where, but I remember reading about this. Based on associated products women buy occasionally they will sometimes get coupons for things like diapers and baby formula while they are pregnant before they know. My guess is that anytime a woman changes her shopping habits the marketers send some stuff her way because even a miss ratio of 90% could still be profitable.

It isn't always accurate, I know a girl that is very not pregnant but gets these things anyway. She babysits occasionally and I suspect that combined with her Baker's card and Walgreen's card give the mass marketing gurus all the information they need to know that she sometimes cares for kids.