I would suggest that you first learn about the indications and contraindications for pregnant women as far as engaging in exercise for that population is concerned.
Then I would look at the organization offering yoga or pilates and see if the guidelines concur.
The best resource for guidelines pertaining to the prenatal population is ACSM Guidelines for Testing and Prescription.
Hope this is of help to you.
I saw in your profile that you are well-credentialed (and a fellow instructor in North Carolina)..
I am not sure how far you want to take the information on prenatal yoga but there is actually an organization here in NC who certifies in that specialty. Here is the link: http://www.theyogaconnectionnc.com/pre_and_postnatal_training.shtml?gcli…
Wish you good luck.