when you are 8 weeks, you have completed your 2nd month. at 9 weeks, you are in your 3rd month, but you arent 3 months pregnant yet because you havent completed the 3rd month. so at 10 weeks you will be 2 1/2 months pregnant. at 9 weeks, you can choose to say either 2 months or 2 1/2 months... since its smack dab in the middle of both answers.