Progesterone Question

Okay, I have been confused about this for almost a year now. Can someone please explain to me how progesterone can maintain and thicken the endometrium during the menstrual cycle but also thin the endometrium when used as contraception? I am struggling to understand how it both promotes pregnancy and prevents pregnancy. (I know it has other effects like thickening cervical mucus, I'm specifically wondering about the endometrial lining!) Thanks in advance!
Source: Student Doctor Network - Category: Universities & Medical Training Authors: Tags: Medical Students (DO) Source Type: forums