Progesterone can prevent pregnancy by itself in several ways:

• Blocking luteinizing hormone (LH) and preventing ovulation
• Maintaining a powerful barrier against the entry of sperm into the uterus by keeping the cervical mucus thick and sticky

Progestins used in contraceptives are referred to as:

• Second generation (levonorgestrel, norethisterone)
• Third generation (desogestrel, gestodene, norgestimate, drospirenone). The third-generation progestins tend to have fewer side effects. Some studies suggest, however, they may pose a slightly higher risk for blood clots than the older progestins.

Combination Estrogen-Progestin Contraceptive Pills

Oral contraceptives that contain both estrogen and progestin are the more common type of oral contraceptive. At least 10 million American women and 100 million women worldwide use combination oral contraceptives. When they were first marketed in the early 1960s, oral contraceptives contained as much as 5 times the amount of estrogen and up to 10 times the amount of progestin currently used. After reports of severe complications (stroke, heart attack, and pulmonary embolisms) in young women, the hormone amounts were significantly reduced.

The estrogen compound used in most oral contraceptives is ethinyl estradiol (also called estradiol, or EE). Fifty micrograms of estradiol is considered high dose, 30 - 35 micrograms are considered average dose, and 20 micrograms or fewer is low-dose. (The high doses found in current oral contraceptives are still much lower than earlier forms of the pill.) Doctors recommend using the lowest possible progestin and estrogen doses. Estrogen doses should not exceed 50 micrograms, as higher doses increase the risk for complications.

Many different types of progestins are used in combination with estradiol. Some common types of progestin, and popular combination oral contraceptive brands, include:

• Desogestrel is the progestrin used in Mircette. Approved in 1998, Mircette was the first oral contraceptive to offer a low estrogen dose and a new type of dosing regimen. Some studies suggest an increased risk for blood clots with desogesterel.
• Drospirenone is used in Yasmin and Yaz. (Yaz contains a lower dose of estrogen than Yasmin.) Because drospirenone increases blood levels of potassium, women should not use Yasmin or Yaz if they have kidney, liver, or adrenal diseases.
• Levonorgestrel is used in Seasonale and Seasonique, as well as many other oral and non-oral contraceptives.
• Norethindrone is used in Loestrin and Loestrin 24 Fe (which adds iron supplements to the placebo pills).
• Norgestrel is used in various generic and brand contraceptives.

Types of Regimens. Combination pills are sold in 21-day or 28-day packs:

• Each pill in a 21-day pack contains estrogen and progestin. Women take 1 pill a day for 21 days, and then wait 7 days before starting a new 21-day pack.
• 28-day packs typically start with 21 hormone pills and add 7 placebo pills that do not contain hormones. After taking hormone pills for 21 days, a woman takes the inactive pills for 7 days. Some newer brands, like Yaz, use 24 days of active pills and 4 days of inactive pills. Mircette uses 21 days of low-dose progestin and estrogen, followed by 2 placebo days, and then 5 days of very low-dose estrogen. Loestrin 24 Fe uses 24 days of active pills followed by 4 days of iron-containing placebo pills.

Oral contraceptives may be taken in cycles that include pills of the same or different strengths. These are categorized as monophasic (one-phase), biphasic (two-phase), or triphasic (three-phase). (In 2010, the FDA approved the first “four-phasic” birth control pill, Natazia.) Monophasic pills contain the same amount of hormones in each dose and deliver the same amount of estrogen and progestin every day. Biphasic, triphasic, and four-phasic pills contain different dosages of hormones within the pill packs and deliver different amounts of progestin and estrogen at two, three, or four times within the 28-day cycle.

Because monophasic pills have a consistent amount of hormones, they tend to cause fewer hormone-fluctuating side effects than biphasic or triphasic pills. However, research shows little difference in effectiveness between these types of oral contraceptives. Monophasic pills are often recommended as the best first-choice for birth control pills.

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Birth Control Options for Women Introduction

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Birth Control Options for Women Implant Contraceptives

Review Date: 09/28/2010
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission (www.urac.org)