Definitely start now!! But personally I found it's quite hard to pick it up from a book/website. I am going to pregnancy pilates and it is covered very well in those classes (not embarrassing at all, the teacher just talks the class through it and nobody can see you doing them!!). If you don't have pregnancy pilates near you, my friend J is going to pregnancy yoga and it is also covered in those classes. Also your midwife should be able to give good advice if you don't like going to classes. Good luck. It REALLY is time well spent, honestly.