Destination Maternity offers prenatal yoga classes but they're definitely not as intense as hot yoga. It's located off of 1-10 and Wurzbach and benefit is that they are free you can look up a class schedule on line.
Destination maternity is where I go regularly in the mornings and on Saturday's. I have done heated yoga in the past, this is a very nice alternative.
Here is the link for the February schedule: http://www.destinationmaternity.com/stores/SanAntonio_February_classes.pdf
My name is Safiyyah, please say hello if you see me, good luck in your search.