I think you will find you're probably more fit than you realise. Running around after little ones is great exercise.
I have got bigger body wise after each pregnancy (and I have had 5!) so I thought I would be terrible when it came time for me to birth DS2, but it was a wonderful birth and then DD2's birth was as close to perfect as you can get (perfection would have been a HB for me)

I am sure your birth will be beautiful honey... just trust that your body knows exactly what it is doing, with or without the aide of yoga