This has to be my favorite subject....
This is posted at least once a month... Progesterone is a funny thing and depending on your Dr. will depend on his/her thoughts.. Progesterone to be effective should be started with 24 hours of ovulation is you have a deficancy. And to help avoid a m/c (IF that is what they think is the cause).. No depending on what you read.. Some Drs think low progesterone causes m/c.. while others feel that the m/c (a pregnancy that is not viable) will cause low progesterone. The creams that you buy over the counter is the least effective. The pill form would be next and the most effective is the vaginal inserts.. (The pill and inserts have to come from your Dr.) All these methods must be stoped to bring on AF (If started prior to your AF) So testing early is a recomendation. Below is a pretty good article. It doesn't take sides....
Progesterone - Using Progesterone to Prevent Recurrent Miscarriage