emergency contraception (evidence relating to efficacy)

Last reviewed 01/2018

• this evidence relates to a reanalysis of the data from the 1998 WHO task force study on emergency contraception.
• the subjects in the study used the Yuzpe method of ethinyloestradiol/levonogestrel (PC4) - 976 subjects, or the levonorgestrel method - 974 subjects.
• the study concluded that (1):
• emergency contraception is associated with a 0.5% pregnancy rate if the first dose is taken within 12 hours of unprotected sexual intercourse.
• the pregnancy rate is 4.1% if the first dose is delayed until 61-72 hours.
• the increase in pregnancy rate follows a linear relationship - there is an approximate 50% increase in pregnancy rate for every consecutive 12 hours' delay in taking the first dose

A more sophisticated estimate of efficacy of the levonorgestrel method of contraception has been undertaken which estimates the proportion of expected pregnancies prevented (2). This measure takes into consideration the fact that, in the absence of emergency contraception only a fraction of women in the study would be expected to become pregnant, ie coitus does not always result in pregnancy:

0-24 hours - 95% efficacy

25-48 hours - 85% efficacy

49-72 hours - 58% efficacy

Reference:

1. Lancet (1999), 353, 721.
2. CMO Update 35 (2003)