**3.3 Mechanism of action**

Depending on their formulation, POPs have several independent modes of action that contribute to their contraceptive effect [51–53]. POPs increase the volume and viscosity of the cervical mucus, preventing sperm penetration into the upper reproductive tract [54]. This change occurs soon after starting a POP, within 2 days of taking the pill. However, the contraceptive effect provided by these mucosal changes is short-lived; it has been estimated that full protection conferred via cervical mucosal changes may last less than 24 h unless maintained by regular pill-taking [53].

In addition, POPs can act to suppress ovulation [54]. However, the extent to which this occurs is variable; for example, up to 60% of cycles in women using LNG pills are anovulatory, whereas ovulation is suppressed in up to 97% of cycles in women using DSG pills [55, 56]. Other modes of action include endometrial changes that hinder implantation and reduction in cilia activity in the fallopian tube that slows the passage of the ovum [54]. The main mechanism of action of DRSP-POPs and DSG-POPs is the suppression of ovulation.
