Progesterone Supplementation During Pregnancy

Don't know if anyone can help me with this...I am currently 11 weeks pregnant. At about 5 weeks I had my progesterone levels checked at which time they were 39. The following week I had them checked again and they came back at 29. My OB said that he saw a "discrepancy" and that it should be around 90! He then offered me Progesterone supplements 100mg a day orally. I have been taking them since then but my perscription runs out at 12.5 weeks. I have been very confused as everything I have read says that progesterone levels fluctuate and as long as they are above 10, there generally isn't anything to worry about. I have chosen not to keep seeing this OB throughout my pregnancy as I don't feel confortable seeing him and am set up to meet a midwife in the next couple of days...does anyone know how long I should be on these supplements for? How do you stop taking them? Do I do it cold turkey? I am very nervous about this as I had a miscarriage back in November. Any advice would be much appreciated!