1) Birth control pills prevent pregnancy through two different mechanisms

All birth control pills use hormones to prevent pregnancy. Some contain a hormone called progestin. Others contain two hormones, progestin and estrogen. All of them work by doing two things: They prevent women from ovulating, and they cause the cervical mucus to thicken, which makes it more difficult for a sperm to penetrate and make contact with an egg if the woman is ovulating.

In a way, birth control mimics the body's response to pregnancy. "There is some truth to the idea that birth control pills trick your body into thinking you're pregnant," said Dr. Vanessa Cullins, Planned Parenthood's vice president of external medical affairs. "When you're pregnant, you don't ovulate, and the cervical mucus is thickening to prevent anything from easily getting into your uterus."

2) Lots of women take birth control incorrectly

If women follow the exact instructions for taking birth control pills — every day, at the same time — they prevent pregnancy in 99 percent of all cases. But lots of people don't do that. In real life, birth control pills have a 9 percent failure rate. That means nine of every 100 women using birth control pills as their only means of contraception become pregnant in any given year.

"It's hard to actually [take the pill at the same time every day] when you're living a busy life," Cullins says. "If you take these pills every single day, the chances of getting pregnant is 1 percent. But typically the chance is much higher than that, because people miss pills. This isn't just true with birth control pills. It's true with any prescription medication."

Birth control pills have a higher failure rate than other contraceptives, like intra-uterine devices (IUDs) or birth control rings.

The main difference: Pills have to be taken every day, which leaves more room for human error.

3) There's a three-hour window for taking your birth control pill "on time"

I asked Cullins whether there is wiggle room in terms of when birth control is effective. For instance, if a birth control user typically takes a pill at 9 am but one morning waits until 11 am, is she at greater risk for pregnancy?

The answer is no. Cullins said that for those taking progestin-only pills, "on time" means taking the pill within the same three-hour window daily. A three-hour difference is not enough to lower the pill's efficacy. "That's acceptable," she said.

For combination progestin-estrogen pills, the space is even wider. Women who miss one day of their pill can take two pills the next day without reducing their birth control's effectiveness. This chart with data from Planned Parenthood shows the organization's recommendations for how to handle a missed combination pill.

"Two or three missed pills is when you need to begin to get concerned, and once you get to three missed pills, you need to consider emergency contraception and using backup birth control until she has finished the first week of the pills of the new package that is begun after her bleed from emergency contraception," Cullins said.

4) Missing a period on the pill doesn't mean something's wrong

Missing a period while on the pill doesn't indicate anything abnormal, Cullins said, as long as you have been taking the pill consistently and correctly each day.

"It's not dangerous not to have your period while on the pill," she says. "What happens is, over time, the uterine lining can become very thin if you take the pill regularly. All that means is if you stop bleeding on the pill, the lining has become so thin that you don't have anything to bleed from."

This is not permanent: When a woman stops taking birth control pills, the ovaries start making more estrogen, the uterine lining gets thicker, and women start to bleed again.

Missed periods after taking your pills incorrectly, however, could indicate a pregnancy. In that situation, it's worth taking a pregnancy test.

5) We don't know whether most antibiotics make birth control less effective

There are two antibiotics that researchers have found make birth control pills less effective: griseofulvin, an antifungal used to treat athlete's foot and ringworm, and rifampicin, which is typically used to treat tuberculosis.

The reason that happens is that these drugs speed up the liver's metabolism, which makes the liver metabolize the hormones in the birth control faster. As a result, hormones leave the blood stream faster and are unable to adequately affect the ovaries to prevent ovulation or the cervix to prevent thickening of the cervical mucus.

Lots of antibiotics, not just the two listed above, come with warnings that they'll make birth control ineffective and suggest using a backup method of contraception. While a backup method is never a bad idea, there's actually sparse evidence that these other drugs make birth control less effective. "Uncertainty persists with respect to the other broad-spectrum antibiotics," researchers in the journal Contraception wrote in a review article about interactions between birth control and antibiotics. They argue that in light of that uncertainty, it is completely appropriate for women to use a backup method — but not to ditch their antibiotics out of concern over interactions.

6) Those "sugar pills" at the end of a birth control pack? They have active ingredients.

Lots of birth control packs have four weeks of pills: three weeks of pills that prevent pregnancy and one week of pills that are inactive.

Women can safely skip that last week of pills and still prevent pregnancy, Cullins said. But that doesn't mean the last week's pills are just sugar pills. As it turns out, some of them actually have active ingredients to make the pills work better or aid in women's health.

"Some of the pills might have low-dose estrogen for three to four days, to help prevent breakthrough bleeding [bleeding in the middle of a cycle]," she said. "Others sometimes contain iron or folic acid or other vitamins. And the hard part about skipping the pills is that you have to remember exactly when to start back up."

7) Even under Obamacare, not everyone with insurance gets free birth control

The number of women getting free birth control pills has quadrupled under Obamacare, recent research shows. Two-thirds of women in a recent Guttmacher Institute survey reported paying zero dollars for their contraceptive.

But that still leaves one-third of women paying something for birth control, even after Obamacare has mandated it be free.

The one-third of women still paying for their birth control are most likely in grandfathered health insurance plans. These are the plans that existed before Obamacare that do not have to comply with the contraceptives mandate (or most other Obamacare requirements, for that matter).

Grandfathered plans are, however, disappearing. When a company significantly changes its insurance (drops a benefit, for example, or changes what enrollees have to pay), then it loses its grandfathered status. Just over a quarter of health insurance plans are currently grandfathered, a number that has steadily dropped since Obamacare passed.

As that figure declines, the number of women accessing no-cost contraceptives will likely continue growing.