Contraception and the menstrual cycle
Contraception is the prevention of pregnancy by inhibiting sperm from reaching a mature ovum or by preventing a fertilized ovum from implanting in the endometrium.
The menstual cycle
The median length of the menstrual cycle is 28 days (range 21–40 days). The first day of menses is day 1. Ovulation usually occurs on day 14. After ovulation, the luteal phase lasts until the beginning of the next cycle.
The hypothalamus secretes gonadotropin-releasing hormone, which stimulates the anterior pituitary to secrete gonadotropins, follicle-stimulating hormone (FSH), and luteinizing hormone (LH).
In the follicular phase, FSH levels increase and cause recruitment of a small group of follicles for continued growth. Between days 5 and 7, one of these becomes the dominant follicle, which later ruptures to release the oocyte. The dominant follicle develops, increasing amounts of estradiol and inhibin, providing a negative feedback on the secretion of gonadotropin-releasing hormone and FSH.
The dominant follicle continues to grow and synthesizes estradiol, progesterone, and androgen. Estradiol stops the menstrual flow from the previous cycle, thickens the endometrial lining, and produces thin, watery cervical mucus. FSH regulates aromatase enzymes that induce conversion of androgens to estrogens in the follicle.
The pituitary releases a midcycle LH surge that stimulates the final stages of follicular maturation and ovulation. Ovulation occurs 24 to 36 hours after the estradiol peak and 10 to 16 hours after the LH peak.
The LH surge is the most clinically useful predictor of approaching ovulation. Conception is most successful when intercourse takes place from 2 days before ovulation to the day of ovulation.
After ovulation, the remaining luteinized follicles become the corpus luteum, which synthesizes androgen, estrogen, and progesterone .
If pregnancy occurs, human chorionic gonadotropin prevents regression of the corpus luteum and stimulates continued production of estrogen and progesterone.
If pregnancy does not occur, the corpus luteum degenerates, progesterone declines, and menstruation occurs.
Goal of Treatment: the prevention of pregnancy following sexual intercourse.
The abstinence (rhythm) method is associated with relatively high pregnancy rates.
Diaphragms are effective because they are barriers and because of the spermicide placed in the diaphragm before insertion. It should be inserted up to 6 hours before intercourse and must be left in place for at least 6 hours after. It should not be left in place for more than 24 hours because of the risk of toxic shock syndrome (TSS).
The cervical cap can be inserted 6 hours prior to intercourse, and should not remain in place for longer than 48 hours to reduce the risk of TSS. A condom should also be used to protect against sexually transmitted diseases (STDs) including human immunodeficiency virus (HIV).
Most condoms made in the United States are latex, which is impermeable to viruses, but ~5% are made from lamb intestine, which is not impermeable to viruses. Mineral oil–based vaginal drug formulations (eg, Cleocin vaginal cream, Premarin vaginal cream, Vagistat, Femstat, and Monistat vaginal suppositories) can decrease the barrier strength of latex.
Condoms with spermicides are not recommended, as they provide no additional protection against pregnancy or STDs and may increase vulnerability to HIV.
The female condom (Reality) covers the labia, as well as the cervix. However, the pregnancy rate is higher than with male condoms.
Spermicides and Spermicide Implanted Barrier Techniques
Most spermicides contain nonoxynol-9, surfactants that destroy sperm cell walls and block entry into the cervical os. They offer no protection against STDs, and when used more than twice daily, nonoxynol-9 may increase the transmission of HIV.
The vaginal contraceptive sponge (Today) contains nonoxynol-9 and provides protection for 24 hours. After intercourse, the sponge must be left in place for at least 6 hours before removal. It should not be left in place for more than 24 to 30 hours to reduce the risk of TSS. It is available without a prescription.
Composition and formulations
Hormonal contraceptives contain either a combination of synthetic estrogen and synthetic progestin or a progestin alone. They also block the LH surge and thus inhibit ovulation. Estrogens suppress FSH release (which may contribute to blocking the LH surge) and also stabilize the endometrial lining and provide cycle control.
Mestranol must be converted to ethinyl estradiol (EE) in the liver to be active. It is ~50% less potent than EE.
Progestins vary in their progestational activity and differ with respect to inherent estrogenic, anti-estrogenic, and androgenic effects. Their estrogenic and anti-estrogenic properties occur because progestins are metabolized to estrogenic substances. Androgenic activity depends on the presence of sex hormone (testosterone) binding globulin and the androgen-to-progesterone activity ratio. If sex hormone binding globulin decreases, free testosterone levels increase, and androgenic side effects are more prominent.