Was to give progesterone support after ovulation IN CASE of pregnancy (natural or IUI/IVF/medicated/etc). Especially for IVF (required). But my OB, when I went in at 5 weeks for beta testing and progesterone tests, explained that if progesterone is low at that point, they would put me on support, however, most studies show that unless progesterone support begins before/around implantation, there is little benefit. Maybe that's why most docs don't prescribe it for natural pregnancies--by the time you've found out you have a little one in there, it's too late for progesterone support to really help. You just have faith that your body made a lining good enough to get the little one burrowed in deep enough to make HCG, it will keep making good stuff happen down there!
Good luck, though