The sustained release offered by contraceptive implants allows for highly effective contraception at relatively low steroid blood levels. Figure 10.6 depicts expected steroid blood levels with implants, injectables, and oral contraceptives. An additional mechanism for contraception was discovered with the antiprogesterone mifepristone (RU486). In the normal cycle, there is a small amount of progesterone production from the follicle just before ovulation. This progesterone appears essential to ovulation, because if mifepristone is given before ovulation this can be delayed for several days (120,121).
Efficacy of Hormonal Contraception
When used consistently, combination OCs have pregnancy rates as low as 2 to 3 per 100 women per year. Progestin-only OCs are less effective than combination estrogen–progestin preparations, with best results of 3 to 4 pregnancies per 100 woman-years. All methods have the potential for user error; therefore, there may be a 10-fold difference between the best results and results in typical users of OCs. Injectable progestins and implants are much less subject to user error than are OCs. The difference between the best results and results in typical users is small and is comparable to pregnancy rates after tubal sterilization (Table 10.2). Pregnancy rates with the OrthoEvra patch and the NuvaRing were equivalent to those of OCs; however, because it is easier to use these methods consistently, larger studies may well demonstrate better typical user results than with OCs (122,123). Typical use pregnancy rates with DMPA are lower. The subdermal implants have the lowest rates of any hormonal contraceptive method.
The present rate of obesity in Europe and the United States is 30% and is increasing. Most studies of contraceptive efficacy intentionally excluded obese women, so the available information is limited. Obese women are no less likely than other women to become pregnant, but they do have an increased risk of pregnancy complications (124). A systematic review of hormonal contraception for overweight and obese women found only seven relevant studies (125). Overweight and obese women appear to be at a similar or slightly higher risk of pregnancy with oral contraceptives. The attributable risk is minimal (124). The efficacy of DMPA does not appear to be reduced in heavier women. A possible increase in pregnancies was reported for users of the contraceptive patch who weight more than 90 kg. Heavier women using the vaginal ring
Figure 10.6 Schematic representation of the expected pharmacokinetic profiles of progestogens administered in different formulations. (From Landgren BM. Mechanism of action of gestagens. Int J Gynaecol Obstet 1990;32:95–110, with permission.) had no increase in pregnancies. Few heavy women were studied with the etonogestrel implant because the studies excluded women of body weight greater than 130% of ideal body weight. Pregnancy rate does appear to increase with weight in women using the levonorgestrel implants at weight 70 kg or more but it is still low. There were no pregnancies in any weight group during the first 3 years of use in a large study (125,126).