Oral contraceptives are the most popular reversible form of birth control today, used by about 30 percent of American women. Introduced in the late 1950s, they found widespread acceptance during the 1960s as a relatively safe and inexpensive form of contraception. For the first time, women had a means of effective contraception they did not need to think about on each and every occasion of intercourse. Some people worried that convenient, reliable contraception would make women promiscuous, and it is certainly true that the availability of the pill did change patterns of sexual behavior.

The pills as first developed contained large doses of estrogen, about 100 µg per pill. Although these high-dose pills often caused bloating, breast tenderness, and depression, many women used them. During the 1970s, high-dose pills were linked to increased risk of blood clots in the veins, as well as heart attacks or strokes, especially for women older than 35 who smoked. Over the years, the estrogen content of birth control pills has gradually been reduced: from 100 to 80 to 50, and now to 30 or 35 µg; a few modern pills contain only 20 µg of estrogen.

Today there are basically two kinds of oral contraceptives: combination pills which contain both synthetic estrogen and synthetic progesterone (progestin) and progestin-only pills, also called minipills.

Combination pills work by blocking ovulation. During the normal menstrual cycle, the rise and fall of estrogen and progesterone levels trigger the monthly release of the egg from the ovary, the thickening of the uterine lining to prepare for a fertilized egg, and the midcycle changes in the cervical mucus that make it easy for sperm to swim toward the egg. During pregnancy both estrogen and progesterone are produced in larger amounts than at other times. Birth control pills that contain these hormones artificially elevate their levels and trick the body into thinking that it is pregnant. The constant high level of estrogen blocks ovulation; the constant high level of progestin inhibits the thickening of the endometrial lining (hindering implantation of a fertilized egg) and keeps the cervical mucus in a state hostile to sperm.

Most pills prescribed today are combination pills. There are several different chemical forms of each hormone, which can be used in different amounts during the month. Ortho-Novum 1/35, for example, has 1 mg of progestin and 35 |ixg of estrogen. Ortho-Novum 7/7/7 has 0.5 mg of progestin for the first seven days, 0.75 mg for the second seven days, and 1.0 mg for the final seven days.

Brands differ in the kind and amount of progestin they contain, and in the kind and amount of synthetic estrogen. The most frequently used form of the latter is ethinyl estradiol. Older pills contained a different synthetic estrogen called mestranol, which is still used in a few brands — for example, Ortho-Novum 1/50. Oral contraceptives differ also in their progestin content. Among the synthetic forms of progesterone developed early in the history of oral contraception are norethindrone and norgestrel. The second and third generations of progestins include levonorgestrel, desogestrel, norgestimate and, in Europe, gestodene. They are all slightly different chemically, and the newer forms seem to have fewer side effects. Although many women notice no difference whatsoever between the varieties, some women respond differently to the various combinations. You may have to experiment to find the best pill for you.

About twenty years ago researchers began looking at the amount of progesterone in oral contraceptives and trying to achieve an ideal balance. If there is too little progesterone in the pill, women may have bleeding or spotting between periods. If there is too much, there may be undesirable side effects, including a lowering of high-density lipoprotein (“good” cholesterol) and an elevation of low-density lipoprotein (“bad” cholesterol).

Monophasic pills use the same amount of synthetic estrogen and progestin all month long. Biphasic pills have two different formulations, and triphasic pills utilize three different proportions of estrogen and progestin during the month. The changing levels of the two hormones more closely mimic their proportions during the menstrual cycle than do the monophasic pills. Some women believe that triphasic pills have fewer side effects, but otherwise one type does not seem to have advantages over another.

Progestin-only pills (minipills), which contain only progestin, are not quite as effective as combination pills, because they do not block ovulation as well. However, women who cannot tolerate estrogen can use them successfully. They inhibit the thickening of the uterine lining, maintain the cervical mucus in a sperm-unfriendly state, and possibly slow the progress of the egg through the fallopian tube.

Table Some Monophasic, Biphasic, and Triphasic Oral Contraceptives

Monophasic

Biphasic

Triphasic

Brevicon Estrostep Ortho-Novum 7/7/7
Lo/Ovral Ortho-Novum 10/11 Tri-Levlen
Loestrin Mircette Tri-Norinyl
Modicon Necon 10/11 Triphasil
Nordette
Norinyl 1 + 35
Ortho-Novum 1/35
Ovcon/35
Yasmin
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