How to know when you are fertile

Biomarkers to pinpoint ovulation and how to find them


Window of Fertility Wilcox 1998 (1).jpg

Outside of the ‘fertile window’, a woman has a 0% chance of conception (source). Medical opinion tells us that our fertile window is about 6 days long.  More specifically, the ideal time for conception is 1-2 days prior to ovulation.  That’s because a woman’s egg typically lives for 12-24 hours and sperm typically live 1-3 days inside a woman’s body (source). Therefore, the highest pregnancy rates seem to be when there is sperm waiting for the egg during this ‘fertile window’ time frame. As you can see in the graph, having intercourse the 1-2 days before ovulation gives you the highest chance of conception. 

In one study published by the Journal of Advanced Nursing, only 13% of women could correctly identify the specific days of menstrual cycle that they are most fertile and could become pregnant by (source). While ovulation typically occurs halfway through your cycle, this varies not only from person to person, but also from cycle to cycle in the same person. So understanding how to use your own biomarkers to pinpoint your individual fertile window can be very helpful.  In this blog post, we will explore the current methods available to identify the elusive fertile window. 


Below are the steps you can take to find your fertile window and the science behind these methods:

  • Track your cervical mucus

  • Track biological indicators such as hormones in your urine or saliva

  • Track your daily temperature

  • Use an app that helps you make biomarker charting easier



As your body prepares for ovulation, the characteristics of your cervical mucus changes. These changes allow the sperm to live longer inside the woman’s body and improve the chances that the sperm will make their way through the fallopian tubes where they can reach the egg. If you are not in your fertile window, your cervical mucus may not allow for the sperm to get through. Egg white or watery cervical mucus is a good sign you are fertile. On the other hand, if your mucus is thick and creamy, that may indicate that you are not in your fertile window (American Pregnancy Association). There are good online resources and trying-to-conceive communities that can help you learn more about using this technique.



Ovulation Predictor Kits (OPK) measure hormonal changes to predict the days before ovulation and the day of ovulation. Products you can use at home typically require either urine or saliva samples.  There is a hormone in your body called the luteinizing hormone (LH) that rises one to two days before you ovulate. Urine ‘pee sticks’ detect LH, and some detect both LH and estrogen. LH typically rises two days before you ovulate and estrogen increases a couple of days before the LH surge. To use OPKs with urine, you urinate on a LH test stick, which then notifies you whether or not there is a surge in this hormone. Saliva tests rely on observations made using a small portable microscope. When it’s time, your dried saliva may form a fern-shaped pattern to indicate that you are fertile.



Temperature is another biomarker to indicate that ovulation is about to occur or has already occurred. A method referred to as the BBT Method measures your Basal Body Temperature (your temperature when you are fully at rest). This requires taking your temperature every morning and charting it either with an app or on paper.  You can use oral or ear temperature thermometers, however, vaginal temperature measurements are the most accurate method to use for determining the temperature changes associated with ovulation. Once you ovulate there is a rise in temperature indicating that you have ovulated. BBT can’t predict your fertile window, but it can give you some sense of when you have ovulated, so that you can estimate when you are likely to ovulate during your next monthly cycle (source).


There are sensors coming to the market that will help pinpoint the elusive fertile window prior to ovulation, which is pretty exciting news for women’s health. These sensors measure vaginal temperatures on a continuous basis, which enable for the most controlled for environment (source). That is because they measure your true core body temperature and are not impacted by external factors such as air temperature.



Apps can also be helpful for estimating a woman’s ‘fertile window’.  These apps will estimate the day of ovulation and the five days before ovulation. To calculate your fertile window using an app, you need to know the length of your last menstrual cycle, beginning with the first day of bleeding to the first day of bleeding with your next cycle. Once you know how many days your last cycle was, subtract 14 days from the anticipated end of your current cycle, and that is the approximate day you will ovulate.


However, each woman’s cycle can vary a good deal from one month to the next, and one study by Dr. Allen Wilcox and team, found that only 30% of women were fertile between days 10 and 17. Some women in the study reached their fertile window much earlier and others much later. This is why you should consider using physical biomarkers of your own body to more accurately pinpoint your own unique ovulation cycle instead of trying to rely on overall averages.



Now that you know more about the methods to detect your fertile window, how much intercourse should you have to maximize your chances of conception? According to the researchers at the National Institute of Environmental Health Sciences, couples who had intercourse every other day during their fertile period had a 22% chance of conceiving in any given cycle, as compared to 25% for couples who had intercourse every day. Couples who had intercourse only once a week reduced their odds of getting pregnant in a given cycle to 10%.

However, the timing of intercourse is not the only thing that affects your likelihood of conceiving.  Other factors such as age, weight, and overall health also play a part in predicting your chances of becoming pregnant. Nevertheless, if you have intercourse every day or every other day during your fertile window, you will certainly increase your odds of conception.