ABSTRACT
The mechanism of action is complex because of variable interactions between the administered progestogen and the endogenous activity of the woman's ovary. Outside of lactation fertile ovulation is prevented in 50%–60% of cycles. In the remainder, there is reliance mainly on progestogenic interference with mucus penetrability. In the United Kingdom, the Oxford/FPA study reported a failure rate for old-type POPs of 3.1 per 100 woman-years between the ages 25 and 29 years, but this improved to 1.0 at 35–39 years of age and was as low as 0.3 for women older than 40 years of age. Realistically, most users are probably not as meticulous as those married middle-class women. Loss of full contraceptive activity through missed pills or vomiting (without successful replacement of the vomited tablet) is believed to start within as little as 3 hours, or 12 hours for the DSG POP.
