As far as I know, no archaeologist has discovered cave paintings from prehistoric women charting their periods on a cave wall. But, I wouldn’t be surprised if they did. Tracking your cycle— counting the days between periods to understand how long your average cycle lasts and when your next period is coming— has been practiced by women for literally centuries. As long as women have been having children, they’ve needed to have some kind of knowledge of what’s going on with their fertility cycles.

Today, fortunately, we have a lot more reliable and accurate methods than counting with a pen and a paper calendar (definitely better than scratching on a cave wall). Period and fertility tracking apps do the calculations and the predictions for you. You input your personal data, and the app will predict when your next period is expected.

While that’s an obvious enough benefit to anyone who’s ever been caught without pads or tampons, there are quite a few other reasons to track your cycle beyond knowing about your next period.

The first day of your most recent period and the average length of your period are some of the first questions a gynecologist will ask. Tracking your period will help you answer these questions more accurately and help you define what is “normal” for you.

It’s also important to remember that everyone has their own cycle that’s unique from everyone else’s. While the global average cycle length is 28 days, anything in the range of 21-34 days is considered normal and healthy for adults (and up to 45 days for teens who have just started menstruating.) This is why what’s normal for you isn’t normal for your best friend or your sister, but is healthy for each of you all the same.

By tracking, you can get a sense of your average cycle length. The length of your cycle also determines when you can get pregnant. There are common misconceptions that you can only get pregnant on your day of ovulation, or that you can get pregnant all the time. Neither of these are true. The time during your cycle when pregnancy is possible is called your fertile window. It’s called the fertile window, because it includes the days prior to, during and after ovulation.

Ovulation divides your cycle into two parts: the “pre-ovulatory” or “follicular phase” and the “post-ovulatory” or “luteal phase”. Because your fertile window happens around your time of ovulation, it includes the last few days of your first phase, and the first day of your second phase.

The pre-ovulatory phase can vary quite a bit for each individual, depending on environmental factors like stress, sleep, exercise, medication, and travel. The global average is 16 days, but it can range from 10 to 23 days. Most of the variance in your cycle is likely to happen in this pre-ovulatory stage.

The post-ovulatory phase of your cycle is much more consistent in length, and only varies by 1-2 days from cycle to cycle. The global average is 13-14 days, but it can range from 10-16 days for some people.

Ovulation occurs at the end of the variable pre-ovulatory phase. This means your day of ovulation isn’t a fixed point in your cycle— it’s a moving target. Your day of ovulation varies by as many days as your pre-ovulatory phase does. This variance is normal, but it makes predicting your fertile window difficult and inaccurate if you try to do it by just counting days.

If you’re trying to get pregnant, you’ll want to be tracking sexual activity in particular during your fertile window. And vice versa if you’re not looking to get pregnant (though we recommend always using another method of contraception besides an app.) Around fertile days, you may experience greater sexual desire and motivation.

Tracking your sexual activity also allows you to notice patterns in your sex drive in relation to your cycle. Tracking your moods, pains and cervical fluid throughout your cycle— especially during your fertile window— will all help you have a better understanding of what an “average cycle” is like for you. Does your mood change depending on where you are in your cycle? Do you get headaches or back pain at the same time every month?

By tracking, you can start to make sense of your individual patterns and, if necessary, plan for them.

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