Contraception

Every method blocks one of a few physiological steps to pregnancy

Pregnancy requires ovulation, sperm reaching and fertilising the ovum, and implantation of the resulting blastocyst in a receptive endometrium. Every contraceptive method can be understood as interrupting one or more of these steps: combined hormonal methods and the progestogen-only injection or implant suppress ovulation via negative feedback on the hypothalamic-pituitary-ovarian axis; barrier methods and sterilisation physically prevent sperm reaching the ovum; the progestogen-only pill and intrauterine devices act chiefly by thickening cervical mucus and altering the endometrium to impede sperm transport and implantation. Grouping methods by mechanism, rather than memorising each one individually, predicts both their efficacy and their side-effect profile.