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Contraception - Background information
How do combined oral contraceptives (COCs) prevent pregnancy?

  • Contraceptive effects
    • Combined oral contraceptives (COCs) act primarily to inhibit ovulation [FFPRHC, 2007b].
      • Ovulation is inhibited by the oestrogen and progestogen components of the COC, which act on the hypothalamo-pituitary axis to reduce production of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). With no surge in LH and FSH to stimulate the ovaries, ovulation does not occur.
    • Seven consecutive doses of COC are required to inhibit ovulation; subsequent doses maintain anovulation [FFPRHC, 2005a; FFPRHC, 2007b].
      • The standard regimen for taking the COC is in 28-day cycles of a pill each day for 21 days, then a pill-free interval of 7 days (or 7 daily inactive pills).
      • The most risky time to miss taking the COC is in the first week of the pill pack, because this extends the pill-free interval.
    • COCs also have contraceptive effects on cervical mucus and the endometrium.
  • Effects on the uterus and menstruation
    • The oestrogen component of the COC causes the endometrium to proliferate and grow.
    • The progestogen component of the COC prevents hyperplasia (excessive growth) of the endometrium by opposing the proliferative effects of oestrogen.
    • The usual 7-day pill-free interval (or 7 daily inactive pills) causes oestrogen and progestogen concentrations to fall, which causes the oestrogen-primed endometrium to slough, mimicking menstruation.

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