You should be able to go for a free trial lesson ("Probelektion" if translation is needed) - do ask. And, any good yoga teacher offering courses for expecting mothers will know which are suitable exercises... Have fun!

By the way, postnatal exercises at the same place as Beate's "Wachsen und Werden" on Neptunstrasse is great too. Beatrice Prader (midwife) gives these and you can take baby along too. Her site's in German, but she speaks English well enough.