How you might be missing your fertile window with OPKs

Once you decide you're ready to get pregnant, you want it to happen pronto. To that end, you’ve likely tried a (sometimes) dizzying array of tools, including ovulation prediction kits (OPKs), apps, and much more to pinpoint ovulation each month.


Thing is, apps and methods that don't rely on your individual fertility cues are merely an educated guess based on averages of past data. Your fertile window and ovulation are unique — and may vary from one cycle to the next, even with regular cycles (1).


The frustrating reality is you may still be missing your fertile window and ovulation despite your best efforts. Why — and what can you do? Here, we'll start with the basics about your fertile window, the life of the egg and sperm, and then explain how OPKs may yield misleading results. We’ll wrap up with a pro tip on timing sex for the best chances of conception.


When is my fertile window?


The American College of Obstetricians and Gynecologists (ACOG) defines the fertile window as the 3-4 days before ovulation (2). Indeed, scientific evidence (basically a mountain of data which has accumulated in the decades since a 1995 study in The New England Journal of Medicine) shows nearly all pregnancies occur in a 6-day period: the five days before ovulation, and ovulation day itself (3).


Influencing your fertile window are factors like the life cycle of the egg and sperm, and the presence of fertile cervical mucus (CM). Sperm, for example, may die within minutes to hours in the vagina’s naturally acidic environment, but may live 3-5 days in fertile CM (4). Fertile CM, described as egg white or watery CM, is alkaline, which makes the vagina friendly to sperm, providing them with safe passage to the egg.


Meanwhile, the egg's life is fleeting. As soon as it leaves an ovary, it has only 12-24 hours to be fertilized (4). Unfertilized, the egg disintegrates and is eventually shed with your uterine lining.


Obviously, these factors make it crucial to identify the days before ovulation. If you're among the many couples using urine-based OPKs to detect the surge of luteinizing hormone (LH) that signals ovulation, you should know your OPK results might be misleading.


LH surge: Is it 16 hours or 48 hours?


First, a bit of background on the LH surge. The day before ovulation, as your brain senses the follicle is ready to rupture and release an egg, your body releases LH. Commercially-available OPKs are known to detect the surge of LH in your urine that occurs 18-24 hours before ovulation (5).


While LH is a highly reliable tool to predict ovulation, LH tests may fail to give you enough notice — because the most fertile days of your cycle are 1-2 days before ovulation (5), plus OPKs have other issues (more on that soon).


Let's break this down to explain the LH surge in greater detail. The LH peak varies significantly, ranging from around 16-48 hours before ovulation, per authors writing for the American College of Obstetricians and Gynecologists (5).


If your LH surge lasts on the higher end, LH tests can work great as long as you have sex ASAP after testing positive. As the authors noted, however, a "significant proportion of women" have already missed the best window for conceiving by the time their LH test detects the LH surge.


What's more, an LH surge may last fewer than 10 hours, so you may miss the surge entirely if you're testing for it only once a day (6). (Test morning and night if you’re worried about missing the surge.)


There's also a problem with false positives with OPKs for women over the age of 40 and with certain conditions. Women over 40 typically maintain a high level of LH throughout a cycle, increasing the risk of a false positive (7). Certain conditions, including polycystic ovary syndrome (PCOS), may also produce a false uptick of LH before the actual surge (7).


Be sure to follow the instructions on when to test carefully. When you start to test depends on your cycle length. For example, if you have a short cycle, you may even need to start testing before your period is even finished.


Ovulation isn't necessarily on day 14 and may be later if you're older


Ovulation trackers typically rely on algorithmic guesses about the average woman's cycle. But studies show most women don't ovulate on day 14 of their cycle (8), and age as well as ethnicity may also impact whether you ovulate earlier or later than the average.


One study showed that the fertile windows of 70% of women did not fall entirely within cycle days 10–17 (8). Another paper showed that 46% of menstrual cycles fluctuated by 7 or more days, and 20% varied 14 days or more (9).


So if you’re relying solely on an ovulation tracker, rather than also tracking fertility cues, you’re risking missing your true fertile window and ovulation.


What's the bottom line?


Ovulating early, late, or somewhere in between doesn't mean you won’t get pregnant. And we’re not suggesting you chuck your OPK in the trash. The trick is to be intimately in tune with your body by keeping tabs on multiple fertility cues, rather than relying on a kit or an app alone.


By tracking your LH, CM, and basal body temperature (BBT), you’ll not only boost your chances of identifying the fertile window before ovulation — you’ll also feel confident knowing you're having sex when you’re most likely to achieve your goal of pregnancy. And one final pro tip: When you get an LH peak, have sex in the next 12 hours if possible, and the next day too.

Want to learn more? Check out this blog about cervical mucus and how it can help you get pregnant.


  1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC27529/
  2. https://www.acog.org/Patients/FAQs/Evaluating-Infertility?IsMobileSet=false
  3. https://epublications.marquette.edu/cgi/viewcontent.cgi?referer=https://www.google.com/&httpsredir=1&article=1309&context=nursing_fac
  4. https://nfp.marquette.edu/reproduction.php
  5. http://npr.pl/badania/timing_intercourse.pdf
  6. https://www.kindara.com/blog/why-you-may-not-be-getting-pregnant-despite-having-sex-in-your-fertile-window
  7. https://www.kindara.com/blog/which-is-more-accurate-ovulation-tests-or-tracking-your-basal-temperatures-and-cervical-fluid
  8. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC27529/
  9. https://www.ncbi.nlm.nih.gov/pubmed/15451332

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