Inclusion Criteria:

• Age 12-25 years at baseline
• Female assigned at birth, with uterus
• Diagnosis of primary ovarian insufficiency or hypogonadotropic hypogonadism, requiring estrogen replacement therapy
• Receiving estradiol therapy-oral (1-2mg) or transdermal (0.05-0.1mg)-for at least 3 months
• Never used progesterone therapy or discontinued progesterone therapy at least 90-days prior to enrollment
• Consents to initiating progesterone therapy

Exclusion Criteria:

• Uterine abnormality (e.g., Müllerian Anomaly, uterine fibroids)
• Inability to characterize the endometrial lining on ultrasound
• History of chemotherapy or radiation therapy
• Inability to complete study questionnaire