Progesterone pill and pregnancy?

Hi. My doc has prescribed a progesterone pill for me to start taking one day after I ovulate. I have low levels of progesterone and, although my cycles are very regular (and I have no problem ovulating) I begin spotting about 7 days before my period begins. I am blessed to have a two year old and he was conceived with no problems, at all. My question: What is your experience with this pill? What is successful for you and did it have any side effects? Did you continue to take the pills through your first trimester and have a healthy, full term pregnancy? Any advice regarding these questions would really mean a lot. Thank you!