Low progesterone?

I have 2 gorgeous boys and hubby and I are trying for our third baby. I never had any issues conceiving previously. We have been trying for over 12 months and I have fallen pregnant twice, but unfortunately lost both of those at around 5 weeks. My doctors don't seem to think there's a problem and are reluctant to do any testing until 3 miscarriages in a row! I've heard that low progesterone can contribute to early pregnancy loss, has anyone had issues with low progesterone? Is it something that I can develop after having 2 successful pregnancies? Or would it have always been an issue? Any info is appreciated.