Study of the Safety and Contraceptive Efficacy of Relugolix Combination Therapy in Women With Uterine Fibroids or Endometriosis Who Are at Risk for Pregnancy

Key Inclusion Criteria: 1. Is a premenopausal woman, 18 to 50 years of age. 2. Is at risk of pregnancy (that is, having heterosexual intercourse at least once per month) and is seeking contraception. 3. Has normal, regular menstrual cycles that are between 21 and 35 days in duration. 4. Has a diagnosis of uterine fibroids or endometriosis meeting either of the following criteria: 1. Diagnosis of uterine fibroids by confirmation of ultrasound performed in the last 2 years and patient report of heavy menstrual bleeding affecting quality of life. 2. Diagnosis of endometriosis and has had surgical or direct visualization (laparoscopy or laparotomy) and/or histopathologic confirmation of endometriosis, and the patient reports moderate, severe, or very severe pain during the most recent menses and/or during nonmenstrual portion of the cycle in the prior month 5. Is willing to use the study intervention as the sole method of contraception for 13 consecutive 28-day treatment cycles and does not intend to use any other form of contraception (for example, condoms). Key Exclusion Criteria: 1. Is pregnant, or breastfeeding, or has breastfed in the last year. 2. Has a known history of infertility or sub-fertility. 3. Has presence or history of a venous thromboembolic event (for example, deep vein thrombosis, pulmonary embolism), an arterial thrombotic or thromboembolic event (for example, myocardial infarction, stroke, or peripheral arterial), or a transient ischemic attack, angina pectoris, or claudication. 4. Has a higher risk of arterial, venous thrombotic, or thromboembolic disorders. 5. Has a history of migraine with aura or focal neurological symptoms. 6. Has uncontrolled hypertension, diabetes with inadequate control, or multiple cardiovascular risk factors. 7. Has a history of clinically significant ventricular arrhythmias. 8. Has clinically significant liver disease, including active viral hepatitis or cirrhosis. 9. Has a history of pancreatitis associated with severe hypertriglyceridemia. 10. Has known human immunodeficiency virus (HIV) infection or high risk of contracting HIV. 11. Has a hepatic hemangioma or has a history of cholestasis with prior estrogen use or during pregnancy. 12. Has a serious contraindication to pregnancy (for example, a medical condition or use of chronic medication such as isotretinoin or thalidomide). 13. History of suicidal ideation or behavior, or confirmed "yes" to any question (with exception of non-suicidal self-injurious behavior, unless deemed as an unacceptable risk by the investigator) on the C-SSRS. 14. Has a bone mineral density Z-score ≤ -2.0 at lumbar spine, femoral neck, or total hip during the screening period. 15. Has a history of or currently has osteoporosis, or other metabolic bone disease, collagen vascular disease, chronic kidney disease (CKD) stage 3 or greater with glomerular filtration rate (GFR) \< 60 mL/min/m2 using Modification of Diet in Renal Disease (MDRD) method, hyperparathyroidism, hyperprolactinemia, known pituitary adenoma, hyperthyroidism, anorexia nervosa, abnormal bone mineral metabolism (eg, hypophosphatemia), or low traumatic (fragility) fracture. 16. Has used chronic glucocorticoids that are oral, parenteral, inhaled (prednisone equivalents of ≥ 2.5 mg daily for ≥ 3 months) in 12 months prior to the study. 17. Has known BRCA mutation or other mutation associated with increased risk of breast cancer.