Is 13 weeks too late to begin prenatal yoga? I haven't done any yoga in over a year!?

I am 13 weeks pregnant, and would like to start prenatal yoga. I had my babies back to back, so I haven't had much time over the past year to take the time to exercise much. Will it hurt me to start yoga now at this stage? I've read in a few magazines that if you weren't used to excercise regimens prior to pregnancy that it's not a good idea to start in the middle of pregnancy. Do you think I'll be okay starting at this point before I get too big? What other techniques are there to stay fit and prepare for the baby's birth at the same time during pregnancy?