The Best Ways To Calculate Your Fertile Days, According To An Expert

When you're trying to get pregnant, knowing the best ways to calculate your fertile days is your key to success. When you're not trying to conceive, understanding your monthly cycle keeps you in touch with your body, and can even be used as a means of contraception (if you're careful and methodical about it). The trouble is that while understanding your fertility is a truly empowering life skill, calculating it yourself involves consistency, precision, and quite a bit of math.

You can get a general sense of your fertile days with an online program — Baby Center offers an ovulation calculator and calendar, for example — or purchase ovulation predictor kits, which test your urine for luteinizing hormone (LH). Mayo Clinic detailed the "cervical mucus method" for those who aren't afraid to get down and dirty, and some women take their basal body temperature for a simple method of natural family planning — but be warned, this method involves graph paper. In the age of smart phones and personal bio-metrics, it's no surprise that there's also an app (several, in fact) for identifying the days you're most likely to get pregnant.

The most heralded of such apps is called Natural Cycles, and Romper reached out to co-founder Dr. Elina Berglund — incidentally, she was a physicist on a team of scientists who won a Nobel Prize in 2013, according to National Public Radio (NPR) — to learn more about how to take charge of your fertility. Natural Cycles is the world's first app to win approval as a contraceptive from European agencies, and now boasts more than 300,000 users.

"Natural Cycles uses a very advanced algorithm that adapts to and gets to know your specific cycle," explains Berglund in an interview with Romper. "Through analysing your basal body temperature and other data you add into the app, like menstruation, the algorithm calculates and pinpoints your fertile window. You can use the information both to prevent or plan a pregnancy." She notes that most women use Natural Cycles for contraception. While the idea of dropping hormonal birth control and quitting condoms is appealing, it also seems risky to me (as it did to Kate Knibbs, over at Gizmodo), though it may simply be that contraceptive habits die hard. (Studies show Natural Cycles is more effective than the pill, as Business Insider reported.) If you're trying to conceive, however, Natural Cycles might just save you a bundle on ovulation predictor kids (OPKs), which don't come cheap.

According to Berglund, "All you need to do is measure your temperature when you wake up and enter it into the app. If you forget to measure, the app will not malfunction — you will just get more red days [or days when you're potentially fertile]." Natural Cycles excels at taking the "human error" out of many of the methods mentioned above. For instance, charting fertility with cervical mucus (CM) alone isn't easy, and CM is only one part of a complex biological painting. Likewise, temperature alone can be hard to chart and understand, and for women with irregular cycles, OPKs aren't always effective. Berglund explains:

"The algorithm behind Natural Cycles is created to get to know your unique cycle, and takes into account sperm survival, variation of your cycle length, ovulation day, temperature fluctuations, length of follicular and luteal phase, and gives green days only when ovulation has been detected, and the ‘non-fertile’ phase of the cycle is entered (as the woman’s fertility changes through her cycle)."

Basal body temperature is the main metric that helps the app — and you — identify your fertile days. (Your baseline temperature rises when you ovulate, and you'll be most fertile just before that rise.) Berglund explains that you can use temperature as "an indirect measurement of the hormones in the female body."

For those intent on conceiving as soon as possible — or targeting their fertile days with defense-missile precision — Berglund recommends combining an app like Natural Cycles with an ovulation kit to "pinpoint ovulation more thoroughly." She notes that it's not necessary, however — the app can identify your fertile days through its algorithm and your inputted information alone.

For women suffering from a hormonal imbalance or major irregularities, Natural Cycles — or any method of calculating your fertile days — probably won't be sufficient, notes Berglund. If you're in that camp, your best bet is to speak with your doctor. A fertility specialist can also help you identify your fertile days.

Smartphone apps, temperature readings, ovulation kits, the cervical mucus method, and even an online calculator can help you understand your cycle with varying degrees of accuracy. If you decide to use one of these methods as contraception, be sure to clear it with your doctor first. If you've been using your knowledge of your fertile days to conceive without success for a while, it's time to visit a doctor, too.

Charting your fertility — through whatever means — can give you amazing insights into the workings of your body. "Today many of us go through life without really understanding how our fertility and body work, and how our cycle affects us," explains Berglund. "I have talked to so many women who get shocked when they find out that you are only fertile six days per cycle. When we are young, we are taught that we can get pregnant any time, and all of a sudden we are 30 plus and want to have kids and then we are told that it is really difficult to get pregnant. Why don’t we talk about fertility in between those years?"

There's an old saying that you should never ignore information about your finances or your health. Knowing when you're fertile, and when you're not, is powerful information that every woman should have, whether she's intent on conceiving, or on staying baby-free a little longer. Because calculating your fertile days on your own is a headache and a half, Natural Cycles may just be the very best way to get the job done.

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