Mechanisms of action include:
(1) suppression of ovulation; (2) a variable dampening effect on the midcycle peaks of LH and FSH; (3) an increase in cervical mucus viscosity; (4) a reduction in the number and size of endometrial glands; and (5) a reduction in cilia motility in the fallopian tube
Combination oral contraceptives - Prevention of ovulation is considered the dominant mechanism of action, with the combination of the 2 steroids creating a synergistic effect that greatly increases their antigonadotropic and ovulation-inhibitory effects; these contraceptives also alter the consistency of cervical mucus, affect the endometrial lining, and alter tubal transport
91-day combination oral contraceptives - Reduce the number of menstrual cycles per year

Benefits of Birth Control Pills
  • Preventing unintended pregnancy
  • Decreasing cramping pain with periods
  • Decreasing blood flow and clots during periods
  • Decreasing risk for uterine cancer
  • Decreasing risk for ovarian cancer by up to 75%
  • Helping with mood changes during the menstrual cycle
  • Controlling the timing of cycles
  • Decreasing facial and body acne





References:
http://emedicine.medscape.com/article/258507-overview
Gynecology | Family Planning | Birth Control and Contraception | Menopause | Dr. Lisa M. Jukes M.D.