Birth control pills come in packs of either 21 “active” pills (containing hormones), or packs of 28 pills, 21 of which are active and seven of which are inactive placebos. The placebo pills are simply a way of staying in the habit of taking a daily pill, even while having your period.
Either package, 21 days or 28 days, can be “monophasic” or “triphasic.” Monophasic pills provide the same dosage level of hormones all through the active cycle, whereas triphasic pills give different dosage levels during each week of active pills. Triphasics were designed to more closely follow a woman's natural hormonal pattern. However, most experts say the fluctuations don't really matter and may even cause extra problems, such as increased “breakthrough” bleeding (sporadic menstruation) while on the Pill, or an increase in Pill-related headaches.
Birth control pills are either called “combined” OCs or “progestinonly” OCs. Combined pills are a combination of the hormones estrogen and progesterone. Progestinonly pills, also called minipills, lack the estrogen component. Since women shouldn't use estrogencontaining products when they are breastfeeding, minipills are often prescribed for women who want protection from pregnancy six weeks after they give birth. Minipills also have lower doses of progestin than combination forms, making them a good choice for women worried about metabolic effects of the hormones (but a bad choice for women who want highly effective birth control). They also require a woman to take them on a rigidly regular schedule.
