Hormonal contraception primarily prevents pregnancy through suppression of the HPO axis. Combined hormonal contraceptives (CHC: pill, patch, ring) contain ethinyl estradiol and a progestin; the estrogen component suppresses FSH and follicular recruitment while the progestin suppresses the LH surge, preventing ovulation. Progestin also thickens cervical mucus (impeding sperm transport), thins the endometrium (reducing receptivity), and slows tubal motility. Progestin-only methods (POP, DMPA, implant, hormonal IUD) rely primarily on cervical mucus thickening and endometrial atrophy, with variable ovulation suppression (DMPA and implant reliably suppress ovulation; POP less consistently). The copper IUD creates a sterile inflammatory reaction in the endometrium, with copper ions toxic to sperm and ova, preventing fertilization. The levonorgestrel IUD releases progestin locally, primarily acting through cervical mucus and endometrial effects. US Medical Eligibility Criteria (MEC) categorize contraindications from 1 (no restriction) to 4 (unacceptable health risk).