Progesterone only for extra "help"?

Kaitlyn • Surprise! Baby number three on the way! Twin sisters who are about to both become big sisters!
Help! My dr prescribed progesterone to "help" an egg attach easily/better even though my progesterone isn't low. She says to stop when/if I get a BFP. This worries me that a sudden drop in progesterone may cause miscarriage? 
Has anyone ever heard of this or had their dr do this? 
​I'm worried it'll be too big of a drop, even though my body is making progesterone on its own.