Assessment of Endometrial Thickness Among Adolescent and Young Adult Patients on Estrogen Replacement Therapy Using Daily Oral Micronized Progesterone Versus the Etonogestrel Implant.

The goal of this observational study is to compare endometrial stripe thickness in adolescent and young adult (AYA) patients with a uterus on estrogen replacement therapy using oral progesterone versus the etonogstrel implant for endometrial protection. The main questions it aims to answer are: Aim 1: Characterize the mean endometrial thickness in AYA on estrogen hormone replacement therapy before initiation of progesterone therapy Aim 2: Characterize the mean changes and variability in endometrial thickness in AYA treated for 6 months with either the etonogestrel implant or continuous oral progesterone Aim 3: Assess satisfaction, side effects, bleeding patterns, any progesterone modifications, and adherence in AYA treated for 6 months with either etonogestrel implant or continuous progesterone Participants will be asked to: * Get two pelvic ultrasounds * Fill out two surveys * Continue their current hormone replacement therapy * Initiate one of two progesterone therapies (prometrium 100mg daily or Nexplanon) Researchers will compare the change in endometrial thickness after 6 months of progesterone use to see if there is a significant difference in the mean change between the prometrium and Nexplanon groups.