Illustration by Elena Scotti/Fusion

Anyone who’s on the pill knows the drill: Take it at the same time every day—or risk pregnancy. Need an extra reminder? Set an alarm. Just to be safe.

But why does taking birth control at the same time make it more effective? And what exactly constitutes “same time”—same time down to the minute? The hour? The general time of day?

Among the millions of American women who take the pill, millions are also likely confused about how, exactly, it works. When taken “perfectly,” the pill is more than 99 percent effective at preventing pregnancies—but when taken “typically,” it’s only 91 percent effective. What does “typically” even mean?

In a hunt for answers, we spoke with physicians about why—and how much—timing matters. Of course, you should always consult with your doctor if you have questions about your personal health. But here’s what we learned about the big picture:

First, a quick refresher on how the pill works

The pill comes in two forms: progestin-only pills, also known as the minipill, and combination pills, made up of both progestin and estrogen. The vast majority of women who take an oral contraceptive take the latter.

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Why these two hormones? When you’re not on the pill, your fertility cycle is determined by the ebb and flow of progesterone and estrogen throughout the month (usually on a cycle of 28 to 32 days). When your estrogen level peaks, your body begins to release an egg—also known as ovulation. In a normal 28-day cycle, ovulation occurs on day 14.

When you consistently take synthetic estrogen, however, this peak never happens—and thus no egg is released. With no egg, sperm have nothing to fertilize.

There’s more: The synthetic hormones also thicken the cervical mucus to stop sperm from reaching an egg.

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So does it matter what time you take the pill?

Not really, if you’re on the combined pill—but you do have to take it daily.

Since the pill relies on hormone levels, it is important to keep those levels as constant as possible, which means taking it consistently. However, if you’re like many women and take the pill at 9 AM one day and 8 PM the next, you’re actually okay, if you’re on combination pills.

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“With combination birth-control pills, taking them at the exact time is not that important,” says Alice Hill, an OBGYN in private practice in Los Angeles. “We tell people to take them at the same time just so they form the habit of doing it every day. But they are still protected as long as they take it daily.”

So taking the pill every day keeps hormone levels high enough to prevent pregnancy. But if you forget to take a pill for over 24 hours (meaning you miss an entire day), doctors recommend that you take it as soon as you remember—even if that means taking two in one day—to boost your hormone levels.

If you miss two or more pills, start taking the pills as soon as you remember, but use a back-up method. And if you're on your last week of pills (right before the placebos), Hill recommends using a backup method "until you are on the next pack." You don't want to miss several days and then go seven days with no hormones with no protection—that could actually put you at risk for pregnancy.

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Here's what generally happens when you miss a combined pill:

Credit: Alejandra Aristizabal

If you’re on progestin-only pills, however, timing is everything. Missing a progestin-only pill by as few as four hours can greatly up your risk of pregnancy, says Hill. This is because progestin-only pills work differently than combination pills: They thicken the cervical mucus and alter the uterine lining, but they don't prevent ovulation the way combination pills do—in other words, an egg can still be released.

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"Since you can ovulate on this pill, you can conceive if more than 28 hours passes in between," Hill warns. "That's why the effectiveness of the combination pill is over 99 percent and the progestin-only pill is 96 percent."

So what exactly counts as 'perfect use' versus 'typical use'?

The definitions remain vague, but here's the gist: "Perfect use" means taking the pill "as directed," according to the CDC, and "typical use" accounts for incorrect or inconsistent use—missing entire or multiple days of pills. Or as Hill puts it: "'Perfect use' means taking the pill daily, not necessarily at the same time. 'Typical use' acknowledges that people forget."

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As we mentioned, the pill is about 99 percent effective under "perfect use" and 91 percent effective under "typical use."

What about that week of placebo pills? How does that work?

With many combination pill cycles, there’s a week every 28 days when placebo pills are taken instead of hormones. But you’re still protected from pregnancy during this week, as long as you’ve taken the previous 21 pills.

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“It takes about seven hormone-free days for an egg to start growing,” explains Hill. “That's why pill packs have seven or fewer sugar pill days. You can't get pregnant on those days.”

However, Hill says the time right after the placebo pills is when women are at highest risk of becoming pregnant. “The first five pills in the package are the most important. If someone picks up their pack late from the pharmacy, they have the highest chance of getting pregnant,” she explains.

Since you’ve just gone seven days without the hormones, suddenly, one or two days can make a big difference—in part because you can begin the process of releasing an egg during this time. Hill says that many women who get pregnant while on the combined pill are under these circumstances. Advice: Pick up next month’s pills before your pack runs out!

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What about antibiotics—do they mess everything up?

Not really. “Antibiotics are metabolized by the liver and may speed up the metabolism of birth control pills, making them less effective,” Hill explains. “This is purely theoretical, though. I've never seen someone get pregnant because of taking antibiotics. The pills these days are so effective that it's really not an issue.”

According to Planned Parenthood, only the antibiotic rifampin, used to treat tuberculosis, has been shown to interfere—“other antibiotics do not make the pill less effective.”

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How is it possible for a woman to get pregnant while taking the pill correctly?

Even when taking the pill in a strictly regimented fashion, it’s still not 100 percent effective. Roughly one in every 100 women who takes the pill perfectly will get pregnant in a year. But why?

Turns out, doctors don’t really know. Cold comfort, I know. Culprits could be metabolism (some women metabolize the hormones faster), weight (Planned Parenthood cautions that the hormones may not work as well in obese or overweight individuals), a recent illness (vomiting or diarrhea can mean your body isn’t absorbing all the hormones), or a combination of issues causing certain women to be the one percent.

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Taryn Hillin is Fusion's love and sex writer, with a large focus on the science of relationships. She also loves dogs, Bourbon barrel-aged beers and popcorn — not necessarily in that order.