Contraception

Contraception works by interrupting one or more steps required for fertilisation: ovulation, sperm passage through cervical mucus, sperm survival and movement, or endometrial receptivity. No method protects against sexually transmitted infections; condoms remain necessary when STI prevention is a goal. Combined hormonal contraception (CHC)—the pill, patch, and vaginal ring—uses estrogen plus progestin to suppress hypothalamic-pituitary-ovarian signalling. Reduced follicle-stimulating hormone limits follicular development, and suppression of the luteinizing hormone surge prevents ovulation. The progestin also thickens cervical mucus and produces an endometrium less favourable to implantation. Estrogen is the component that drives most arterial and venous thrombotic concern. Progestin-only methods do not all behave identically. The implant produces a sustained progestin effect that usually suppresses ovulation and thickens cervical mucus. Traditional progestin-only pills depend heavily on maintaining cervical mucus changes, so timing matters. The drospirenone 4 mg pill has a longer missed-pill window than traditional three-hour pills. The levonorgestrel intrauterine device (IUD) acts predominantly within the uterus by thickening mucus and altering the endometrium; ovulation may continue. The copper IUD creates a local environment that impairs sperm function and fertilisation without...