Is your progesterone low? If it is, I'd ask for a rx from your doctor. When I was ttc, my OB prescribed it to start 3 days after ovulation (just to make sure I had actually ovulated b4 I started it) and then stop it if/when my period started. I did use the otc cream in my 1st pregnancy bc my progesterone was low (it was 7 and my hcg was over 30,000) and my OB wouldn't prescribe it at that point bc his belief is that low progesterone at that point (once you are already pregnant) is an indication of a nonviable pregnancy. It didn't help of course bc it was a nonviable pregnancy and the otc stuff isn't nearly as strong as the prescription, which wouldn't have helped either at that point. Best of luck.
edit: Adding: I had 3 miscarriages before I had my daughter, and that pregnancy was a total surprise, I wasn't trying and wasn't on progesterone.