Is It Safe To Take Progesterone As Soon As The Ovulation Starts?

Hi, A little background about myself. I am 26 years old, 5'10", and 165 lbs. I have depression but have been off of my medication (Zoloft) for about 1 1/2 months now. My question concerns my 2 recurrent miscarriages: 1st at 11 weeks (missed m/c - baby died at 8 weeks 5 days) and the 2nd just days after a positive pregnancy test. My doctor tested me for APS and my blood work came back normal. He told me to call as soon as I get a positive pregnancy test and he would start me on progesterone, but he hasn't tested to see if my levels are even low. I guess my question is this: Shouldn't I start taking progesterone as soon as I ovulate instead of waiting for a positive test if he is concerned that my levels are low? Naturally I am very worried to try this again, so any advice will be appreciated.