The purpose of this study is to characterize the longitudinal bone mineral density (BMD) in premenopausal women with uterine fibroids or endometriosis.

This is a prospective observational study to characterize longitudinal BMD of premenopausal women with uterine fibroids or endometriosis over the 52-week observational period.

Approximately 660 participants will be recruited into two cohorts: (1) premenopausal women with uterine fibroids confirmed by an ultrasound (approximately 260 participants), and (2) premenopausal women with endometriosis diagnosed or confirmed by surgical or direct visualization, or histopathology within 10 years of the Screening visit (approximately 400 participants).

Since age is a strong risk factor for BMD change over time, participants in this observational study will be matched by age category (18 to 24, 25 to 34, 35 to 44, and ≥45 years old), with participants enrolled in the interventional studies of relugolix.

During the Screening/Baseline period, the participant should be assessed for eligibility, and baseline dual-energy X-ray absorptiometry (DXA) scan will be obtained. Bone densitometry (lumbar spine [L1-L4], total hip, and femoral neck) will be obtained and submitted for central reading at Baseline, Week 24, and Week 52.

Information related to health care utilization for uterine fibroids or endometriosis and concomitant medications will be collected at every visit and during the phone calls at Week 12 and Week 36.

Collection of adverse events in this study will be limited to protocol-specified procedure-related adverse events. Reports of pregnancy will also be collected.

Inclusion Criteria:

1. Is a premenopausal female 18 to 50 years old

2. By the participant's report, had two consecutive regular menstrual cycles immediately prior to enrollment in the study. Participants with hormonal intrauterine devices or on other hormonal contraception methods that disrupt cyclic bleeding are not required to have two consecutive regular menstrual cycles.

3. Has one of the following conditions:

   1. Diagnosis of uterine fibroids that is confirmed by an ultrasound within 5 years prior to screening.
   2. Diagnosis of endometriosis confirmed by surgical or direct visualization and/or histopathologic confirmation.

4. Has not and is not expected to undergo a surgical procedure with bilateral oophorectomy within the 12 months following enrollment.

   Exclusion Criteria:

5. Has a weight that exceeds the weight limit of the dual-energy X-ray absorptiometry scanner

6. Has a baseline BMD z-score < -2.0 at spine, total hip, or femoral neck

7. Has a history of or currently has osteoporosis, or other metabolic bone disease

8. Use within the past 3 months prior to Screening or anticipated use of systemic glucocorticoids

9. Is currently pregnant or lactating, or intends to become pregnant during the study period

Women with endometriosis or with uterine fibroids