Q: Why can't an app work for birth control when you have irregular cycles, but Fertility Awareness can?

(Read on, or if you'd like to hear me explain in a video, click here.)

A: The answer has to do with relying on prediction, versus real-time information. An app that calculates fertile days based on past cycle lengths and current Basal Body Temperature (BBT) – even one that uses a sophisticated algorithm – relies on some PREDICTABILITY to the menstrual cycle. This is also true of any natural birth control method that uses calculations, such as the Standard Days Method, or the old rhythm method.

If you're using a Fertility Awareness method that follows day-to-day changes in cervical mucus, it doesn't matter whether your cycle looks anything like the cycles you've had before.

Remember that cervical mucus tells you ovulation is APPROACHING. Sperm can survive in that cervical mucus, waiting for ovulation. BBT shift (your temperature rise) tells you that ovulation has PASSED.

Once you learn how to check for cervical mucus AND know what to pay attention to about your cervical mucus, you can always be confident about which days are fertile and which days are infertile – no matter how long your cycle is, and no matter how many times in one cycle cervical mucus comes and goes.

(I know there can be a lot of confusion about cervical mucus, other vaginal discharge, and what changes in cervical mucus you need to track. The Justisse Method is terrific about making this all clear.)

Remember that an app relying on past cycle lengths and BBT data PREDICTS when you are going to ovulate. Before ovulation, it can only calculate your LIKELY fertile days based on past data. Only after your BBT rises can you be sure you're not fertile.

Those apps can be pretty accurate while your cycle is regular. But please also remember that every woman has some variation in her cycle length from time to time. It's common to have shorter cycles than you're used to during perimenopause, which usually begins when you're in your late 40's but can begin much earlier, sometime in your 30's.

When a cycle is shorter than you're used to, your fertile days come EARLIER than you're used to. This is a time when a form of natural birth control that relies on predictability could fail you. (Those are still fine options for you, of course, if that's what suits you best! I'm all about FULLY informed choice, so I want to make sure you know what you're choosing.)

Cycles can also be longer than you're used to, especially during a time of hormonal change. At those times, your app might consider many MORE days to be fertile than you really need to, which isn't the best if you're waiting for infertile days to have sex.

In fact, considering more days fertile than you really need to is the way apps can be accurate at all about irregular cycles.

The best way I know of to be confident that you're keeping up with any changes from one cycle to another is to track your cervical mucus in real time – whether you're used to regular or irregular cycles.