Progesterone

Jennifer
Has anyone taken progesterone supplements? I went to the doctor last Thursday and they sent me for blood work. It came back saying my progestin level was at a 5. It's supposed to be around 15. The doctor told me that they could give me progesterone supplements to make sure my levels stayed up to avoid a misscarraige. 
Does anyone have experience with this? This is our 4th child and I've never dealt with this before.
Thanks!