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How do birth control pills really work, even during the placebo period?

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curiousgirl asks:

I have been on the pill for over 2 years, and have never missed a pill. I just started my inactive pills on Sunday of this week, and had sex with my boyfriend on Monday. He wore a condom but I am still concerned there is a chance I might still get pregnant. Is it possible for this to happen or am I safe?

Heather Corinna replies:

So long as you take your pills as directed and on time, with the exception of a few medications or herbs that can interact with the pill, you're as covered as you get.

We get questions like this a lot, so in the interest of making sure you and everyone else understand your pill as best you can, let me explain how the pill works and thus, why it works just the same during the placebo period (the time you take or have the inactive pills) as it does during the other three weeks of your pill cycle.

If you understand how your menstrual and fertility cycle works when you aren't on oral contraceptives (and if you don't, read this before reading further), it's easier to understand how the pill works.

During a typical, unmodified fertility cycle, everything that happens is first driven by the signals your brain, via the hypothalamus and your pituitary gland, sends your reproductive system. Based on those actions, the release of an egg by the ovary during a normal cycle also releases hormones which direct the rest of the cycle, including the type of vaginal secretions you'll have for the best sperm mobility during the fertile period.

When you menstruate, it's because the egg released by the ovary either wasn't fertilized, or was fertilized but did not implant in the uterus, so your endometrium which has built up from all of those directives, to prepare for a fertilized egg implanting, then sloughs itself off. That's what your flow is during menses (which is also why sometimes it has little globs in it, rather than just fluid).

When you take the pill, the synthetic hormones send a different set of signals to your reproductive system entirely, so that you do not ovulate -- release that egg -- so that your vaginal secretions become and remain thicker (to make it tougher for sperm to get to an egg in the case something went amiss there), and so that that endometrial lining doesn't build up as much (in the case that the other two modes go awry, that would make it really tough for a fertilized egg to implant). That's three different ways to protect you from pregnancy, and even just one of those ways is often enough.

The reason why you don't have any extra risks during that placebo week is because of all of the things the pill has done in the three weeks prior, and which it will do once you start taking it again. During that week, you don't need pills because they've already prevented ovulation and fertilization, so you couldn't become pregnant during that period of time, as without all those preceding signals to be fertile, you can't suddenly become fertile in that week.

Sometimes this is more simply explained by saying that the pill tricks your body into thinking it already ovulated in a given cycle -- thinking that through every day of every cycle you're using the pill. In a normal fertility cycle, once ovulation has come and gone, a person is highly unlikely to become pregnant. Same goes for people using the pill.

For more on how the pill works, one of my favorite explanations in visual format is on the PBS site about the pill, here. The Feminist Women's Health Center also has a great general page about the pill, including loads of extra links.

While there are certainly downsides and risks with taking the pill, one thing that isn't a downside is how incredibly effective it is: in perfect use, the pill is very close to 100% effective. When you combine it with condom use -- -- which is always wise, since the pill offers no STI protection -- which are around 98% effective in perfect use, you have an awesome combined method that is beyond unlikely to fail.

Just be sure that you are taking your pills as directed -- and if you've lost your pill insert and need a reminder on what that means, check this out -- and on time, and it's all good. Keep combining your pills with condoms and I really can assure you that there is just no sound reason to be worrying yourself about pregnancy. Obviously, no method, even a combined one, is 100% effective, but right now, you've got as close as it gets.

written 17 Oct 2007 . updated 21 Jan 2014

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