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Getting Pregnant

Is Your Ovulation Tracker Wrong? How to Really Know When You’re Fertile

Once you’ve decided you want to conceive, you probably want it to happen quickly. To that end, you might be using an ovulation tracker to find your fertile window.

There are a variety of tools (apps, fertility calendars, ovulation calculators, and ovulation tests) that promise to pinpoint ovulation, and therefore your fertile window. However, the reality is that the timing of ovulation is unpredictable—even if you have a regular cycle.

Studies show that most women don’t ovulate on the fourteenth day of their cycle, as they are commonly told. (In fact, one study showed that 70% of women have fertile windows that don’t fall entirely within cycle days 10 – 17.)

So, if your ovulation tracker follows the “ovulation occurs on cycle day 14” formula, then chances are it’s not accurate. Here, we’ll review:

  • When ovulation happens
  • How to find your fertile window
  • The best time to get pregnant—backed by research

Why it’s important to know your fertile window when TTC

Each menstrual cycle, you have a six-day fertile window: the five days before ovulation, and ovulation day itself. A study published in The New England Journal of Medicine  showed that nearly all pregnancies occurred within a six-day period which ends on the day of ovulation.

When you’re trying to get pregnant, it’s important to know when you ovulate, yes. But for the best chance of conception, you really want to narrow in on those days before ovulation. These are your most fertile days—and the days that you want to be having sex.

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When do you ovulate?

The short answer is: it’s variable. (Which is why most ovulation trackers won’t work.) Ovulation is a complex hormonal process driven by four different hormones: Follicle stimulating hormone (FSH), Luteinizing hormone (LH), estradiol, and progesterone. The critical thing to figure out is when ovulation happens for you since it doesn’t occur according to a schedule and can even change from cycle to cycle.

First, let’s review the ovulation process itself. Follicle stimulating hormone (FSH) will trigger the growth of multiple follicles inside your ovary. Relatively early in your cycle, one of these follicles will become dominant and the others will die out. That dominant follicle produces increasing levels of estrogen, which causes your cervical mucus to change consistency and pH balance and your uterine lining to thicken and grow. Estrogen then continues to rise—and once it’s high enough— it triggers the release of LH, which sends the signal for ovulation to occur.

The follicle will rupture and a mature egg is released—this is ovulation!

What you’ve likely heard is that ovulation occurs 14 days after your period starts. This is a myth that even some healthcare professionals still believe. But recent research has shown that only 10% of women actually ovulate on day 14 of their cycle, which certainly isn’t anything close to the majority. Most important takeaway: ovulation follows a series of hormonal events that happen each month, but the pace at which it unfolds can vary.

Why you might be missing your most fertile days

There is a wide range of products promising to help you identify your fertile window. But it’s important to note that not all of these tools are created equal. In fact, this study found that the large majority of fertility apps are not based on evidence-based FABMs (fertility awareness-based methods).

Some fertility trackers use your average cycle length and an estimated timeframe for ovulation to pinpoint when your fertile window might be. But your peak fertility only lasts for about three days. So using averages and estimations—even if they’re only off by a few days—may cause you to miss your most fertile days completely.

Which days are those, exactly? According to three different studies (each with slightly different findings detailed in our fertility calendar), here are your most fertile days, ranked in order:

2 – 3 days before ovulation  20 – 30 percent chance of conception
4 days before ovulation
10 – 12 percent chance of conception
ovulation day
10 – 12 percent chance of conception
5 – 6 days before ovulation
0 – 5 percent chance of conception

The most important thing to note here is that you can conceive anytime during the 5 days before ovulation and on ovulation day itself. Within your 6-day fertile window, the 2-3 days before ovulation are your most fertile.

Can you get pregnant on ovulation day?

Yes, but your chances are better in the days leading up to ovulation. As the table above shows, there are varying chances of conception on each day, but conception can happen anytime in the fertile window.  The good news is that sperm can live inside the female reproductive tract for up to five days. According to this study, millions of sperm are deposited into the vagina during sex, but only a few thousand of them will reach the Fallopian tubes.

These remaining sperm will wait for ovulation to occur and once it does, will fight their way to the just-released egg. Just how the sperm knows that ovulation is happening and it’s time to get going is still a bit of a mystery.

One study showed that sperm may be drawn to the egg through a process akin to “smelling,” but more research is needed to figure out exactly how this fascinating process works. But the point is: somehow when the egg is released, the sperm know they have an important job to do.

What are the best options for tracking ovulation?

The good news is, there are a number of reliable ways to narrow in on your fertile window. Here are some options for tracking fertile days:

  • Cervical mucus—A few days prior to ovulation, your cervical mucus will begin to become slippery, resembling egg whites. This is a symptom of ovulation and a sign that you are fertile. When you see discharge looking like this, take it as a sign that you’re in your fertile window.
  • Ovulation tests—An ovulation test (also called an OPK for “ovulation predictor kit”) detects the concentration and presence of luteinizing hormone (LH) in your urine. Around 12-36 hours prior to ovulation, there’s a surge in LH levels. This surge can last anywhere from a couple of hours to a couple of days. The length of the surge isn’t important in determining when ovulation happens, as most women tend to ovulate about 24 hours after the surge begins. Unfortunately, LH tests can’t tell you if your surge just began or if it’s nearing the end. The problem for TTC? By the time you get a positive LH test, there may not be much (or any) time left before ovulation—and you may have missed some of your most fertile days already, depending on the timing of your LH surge.
  • Basal body temperature—Tracking your basal body temperature (or BBT) is another option. With this method, you take your temperature at the same time each morning—before you get out of bed or move around at all. You also need to use a special thermometer specifically designed to measure basal body temperature. The problem with the BBT method is that your basal body temperature rises after ovulation. So once you detect that temperature increase, your fertile window for that cycle has already closed. 
  • Ava—This wearable device is clinically proven to detect the very beginning of the fertile window—in real time, as it happens. You wear the bracelet each night while you sleep.  Ava’s proprietary algorithm uses machine learning and artificial intelligence, combining knowledge of your cycle with the physiological data collected (parameters like resting pulse rate, breathing rate and skin temperature, to name a few) to predict your fertile window accurately.


View sources

The timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective study

The Performance of Fertility Awareness-based Method Apps Marketed to Avoid Pregnancy

Sperm transport in the female reproductive tract

Identification of a Testicular Odorant Receptor Mediating Human Sperm Chemotaxis

Timing of Sexual Intercourse in Relation to Ovulation — Effects on the Probability of Conception, Survival of the Pregnancy, and Sex of the Baby

Lindsay Meisel

Lindsay Meisel is the Head of Content at Ava. She has over a decade of experience writing about science, technology, and health, with a focus on women's health and the menstrual cycle. Her work has been featured on The Fertility Hour, The Birth Hour, The Breakthrough Journal, and The Rumpus.

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