Hi Jamie
Sorry I'm coming in to this a bit late, but I'm just back from India (studying Yoga of course). I teach prenatal Yoga and have seen again and again the benefits of these classes in pregnancy.
One of my students, a ward sister in the local hospital who came to me with serious anxiety issues due to 3 miscarriages, went so far as to say that she couldn't see how anyone could do it without the Yoga practices (her husband then pointed out, "Well you do know how they do it, you heard the girl screaming in the next room"). It only took me one class to turn her around on the anxiety front, she enjoyed her full-term pregnancy and now has a beautiful, healthy baby boy.
As far as when to do it, I think the general consensus in here is absolutely correct - sooner rather than later. The practices must be PRACTICED to be of any use to you. Explain your situation to the teacher in advance, that you can only afford one block - it may be a bit embarrassing but it may benefit oyu on two fronts. First, that teacher should then make sure that during your 6-week block s/he covers everything you need to know. Second, you may find (since most of us do this from a desire ot help, not for money alone) that the teacher may be able to help you further - perhaps chargin you less, or teaching you privately whenver s/he has free time.
Hope that's of some help,
Scott