This cross-sectional comparison and prospective cohort design study will investigate differences in calcium metabolism, biochemical markers of bone and reproductive health, musculoskeletal health, and iron status between women using different hormonal contraceptives (combined oral contraceptive pill (COCP), hormonal implant, hormonal intra-uterine system (IUS), hormonal contraceptive injection, and eumenorrheic non-hormonal contraceptive users). The same outcomes will also be examined across a menstrual cycle in the eumenorrheic non-hormonal contraceptive users. The study will test the following hypotheses: Hormonal contraceptive use 1. Biochemical markers of bone resorption and formation and ratio of urinary 44Ca:42Ca will be higher in the implant and injection groups compared with IUS (which exerts localised effects) and non-HC users (ovulatory phase), and lower in COCP compared with non-HC users; 2. Oestradiol and progesterone will be lower in hormonal contraceptive users compared with non-HC users during the ovulatory phase; 3. Bone macro- and microstructure, muscle strength, and tissue properties are different in hormonal contraceptive users compared with non-HC users; 4. Calcium and bone metabolism, reproductive hormones and musculoskeletal function are different between the pill phase and non-pill phase of COCP use. Menstrual cycle phase 1. Calcium and bone metabolism are lower during the ovulatory phase compared with menses, mid follicular and mid luteal phases. 2. Muscle strength and tissue properties are different across the menstrual cycle in non-HC users.
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Calcium balance.
Timeframe: Baseline measurement for long acting reversible contraceptive users. Day 21 and 28 of the pill cycle for the combined oral contraceptive pill users. Menses (day 0), mid-follicular, ovulation, and mid-luteal phase for non-users.