Franch, I go to yoga base on Monday nights and yoga therapy on Wednesdays. Yoga base prefer you to book a block of 5 or 6 classes at once but the teacher is quite flexible and will let you try one class out to see if you like it first. She will also sometimes let you pay on a class by class basis if you know you're going to be away during your block.
Yoga therapy seem much more flexible and you can just drop in and pay for one class or pay for a block (which don't have to be consecutive if you know which weeks you'll miss) Yogatherapy also do separate classes for women below 30 weeks and for women over 30 weeks. On wednesdays the under 30 weeks class is at 7:30. 6:15 for over 30 weeks.
Now IMO there is a huge difference in the two classes. The Yogabase class is a bit more new age but not over the top. She usually has incense burning, there's lots of ommmmmmming and things like imagining a golden ball of light around you. Also the class is very friendly and chatty. But there's not a lot of discussion of birth positions or explanation of what the exercises are doing.
The Yogatherapy class is a lot different. It doesn't feel so New Age (to me) but the teacher really explains what the different exercises are about. For example, how they help make room for your baby, how they will help open your pelvis, strengthen your pelvic floor etc. She seems to know a lot about pregnancy and childbirth and makes the whole experience feel very positive and special. The teacher is actually the founder of birthlight. (website at birthlight.com) The class maybe isn't as chatty as the Yogabase one, but that could have been just my first impression.
If you had to choose between the two, I would go to Yogatherapy because the teacher really is something special. (Not that the yogabase teacher isn't) I think I just connect more with the Yogatherapy one. I'm going to try to keep going to both though.
Hope that helps, if you need anymore info CAT me.