Low Progesterone- 5 weeks

Just got a call from my OB and my beta hcg is 4800 but my progesterone is only 8.1. She said they like to keep it around 14 so I have to take progesterone supplements for my 1st trimester. Has anyone else done this and can share a good experience with a healthy pregnancy? What does low progesterone mean? I'm soooo nervous!