Question for Progestin Test

Hi all, I understand that if there's bleeding after progestin withdrawl, then that indicates anovulation (low progesterone) as the cause of amenorrhea. My question is, why does low progesterone cause amenorrhea? In my mind, it'd cause heavy menstration since you can't maintain the endometrial lining formed by estrogen. Thanks for your time!
Source: Student Doctor Network - Category: Universities & Medical Training Authors: Source Type: forums