For example, women who suffer repeated first-trimester miscarriages may not be producing enough progesterone, a hormone that is critical for sustaining pregnancy.

If you don't produce adequate levels of progesterone on your own, your uterine lining, where the baby grows, might not be able to sustain your pregnancy.

http://www.fitpregnancy.com/pregnancy/getting-pregnant/secret-loss

http://www.totalityofbeing.com/FramelessPages/Articles/IncFertility.htm