2nd miscarriage low on progesterone

I just had miscarriage #2 since May. Found out yesterday I am low on progesterone and my Dr prescribed me suppositories to increase my level, but it was too late my hcg levels already dropped.

I see on here some ladies take the progesterone before they are even pregnant.

When do I know when to start them again? Anything else I should get tested for to prevent a 3rd? Thanks in advance.