What is the purpose of estradiol and prometrium after IVF?

I am 9 weeks pregnant from IVF. I have instructions to stay on estradiol and prometrium until the 12th week. What is the purpose of these meds. I have placed a call to see if I can ween off of the meds earlier. I think the meds make me extra nauseous and are making this first trimester even tougher. I wonder if the meds are a precaution or absolutely necessary, and if I can decrease the meds earlier. Thank you.