Women who smoke and are older than 35 should not take any type of combination birth control pill, according to the FDA. Other factors - such as being obese or having a family history of heart disease - also make those clots more likely. So consider your overall risk.

Still, those clots are rare, overall.

“Somewhere on the order of two to four more women per 10,000 might experience a nonfatal [blood clot],” says Sarah Prager, MD, MAS, an assistant professor of obstetrics and gynecology at University of Washington Medical Center.

The FDA advises women not to take any combination birth control pills if they have a history of blood clots, heart attack, or stroke.

The FDA has reviewed recent observational studies on whether certain combination pills that contain drospirenone (a synthetic version of progesterone or a progestin) carry a higher risk than other pills of deep vein thrombosis (DVT) or Pulmonary embolism - two rare but serious blood clots. Based on that review, the FDA states that it has concluded that drospirenone-containing birth control pills “may be associated with a higher risk for blood clots than other progestin-containing pills.” Pills containing drospirenone include Beyaz, Gianvi, Loryna, Ocella, Safyral, Syeda, Yasmin, Yaz, and Zarah.

Progestin-only pills (also called the “mini pill”). These pills are most commonly used by nursing mothers, women with pre-existing risks for blood clots, or other conditions that prevent them from taking estrogen.

If you don’t take them at the same time every day, they may not work. Being as little as three hours late could result in ovulation.

This type of pill works for nursing mothers because continuous breastfeeding already protects against pregnancy, and the mini pill simply provides added security. “Nursing moms have lower fertility. The mini pill’s efficacy may be unacceptably low in women who have normal fertility,” says Andrew Kaunitz, MD, who is associate chair of the ob-gyn department at the University of Florida College of Medicine in Jacksonville, Fla.

What’s in That Pill Packet?

Combination birth control pills come in different phases, depending on whether the level of hormones in the pills changes throughout the month.

- Monophasic (one-phase) pills contain the same amount of estrogen and progestin in all of the active pills. Alesse, Loestrin, Ortho-cyclen, Seasonale, and Yaz are a few examples. Each active pill in the pack is the same. If you forget to take a pill one day, you take it as soon as you remember, and then take your next pill at your regularly scheduled time.

- Biphasic (two-phase) pills change the level of hormones estrogen and progestin once during the menstrual cycle. Examples include Kariva and Mircette Ortho-Novum 10/11.

- Triphasic (three-phase) pills contain three different doses of hormones in the active pills. Those levels change every seven days during the first three weeks of pills. This was the first type of birth control pill. Examples include Cyclessa, Ortho Tri-Cyclen, Nortel 7/7/7, Enpresse, and Ortho-Novum 7/7/7.

- Quadraphasic (four-phase) pills. The hormone levels in these pills change four times per cycle. Natazia is the only quadraphasic pill on the U.S. market.