I took progesterone this pregnancy until 11 weeks, and had two miscarriages last year (no living children). I rechecked the levels myself after stopping and they were in the lowish range according to the lab, and in zone 2 according to the Napro chart. I decided I couldn't deal with that level of stress with monitoring without compelling evidence that it actually works, and I didn't want to be taking the supplement anymore if I didn't need to. Napro says they have studied it, but they don't say what those charts are based on, and only provide anecdotes which aren't terribly convincing either. I am super interested in this area of information, but it just doesn't seem to be growing. If they could make a clearer distinction between progesterone levels that are problematic and those that aren't, that would be nice. However, I know women who have had successful pregnancies with low progesterone throughout. It just isn't well understood, and I didn't want to perpetuate any feelings of "needing" a technology when I likely did not. So far so good at 21 weeks, but I can't say I'm not still nervous. The placenta should be making plenty of progesterone though, and there is really no reason it wouldn't, unless the pregnancy wasn't doing well.