Congratulations!!
I will start student teaching this January - my daughter turns one year old during my 2nd week! While you will face extra challenges, it can be done. Especially if you aren't due until well after you are done. Teachers teach pregnant all the time...
While this is your decision to make, if it were me - I would tell my University supervisor and cooperating teacher ASAP! They need to know so they can accommodate you. You may be ill, need to stay off your feet, use the restroom frequently, you will not be able to lift anything over 10-20lbs, etc. The sooner they know, the better prepared you all will be for any obstacles you may face. It will be better if you all start out on the same page, and are completely open with each other.
Best of luck to you and I hope you have a happy healthy nine months!!!