When are you meant to book them? My midwife told me about them at my booking appointment and something about ringing someone (I don't know who, I just have a phone number) at 16 weeks. However, all the antenatal classes that I have heard about (admittedly not in my area) appear to start around 30 weeks. I'm terribly confused! Do you think I should just give them a ring tomorrow and sound like an idiot when she says I'm a bit eager at 20 weeks?! 
