Ovulation Calculators, What to Know When Trying to Get Pregnant

 "One of the #1 reasons my patients aren't getting pregnant as quickly as they would like to, is that they are not timing their fertile window correctly."- Dr. Don Aptekar M.D., FACOG
Fig. 1

Fig. 1

Timing your fertile window is one of the most important steps you can take to get pregnant. Outside of your fertile window, there is nearly a 0% chance of getting pregnant (see Fig.1). 

The fertile window is the 5 days before ovulation and the day of ovulation. Many women use an online ovulation calculator to find this elusive window.  The issue with ovulation calculators is that the dates provided to you are a guess based on averages of when women ovulate. While it is one of the easiest and least expensive methods, the disadvantage is that you sacrifice accuracy and you may miss your optimal time for intercourse.     

In only about 30% of women is the fertile window entirely within the days of the menstrual cycle identified by clinical guidelines—that is, between days 10 and 17. Most women reach their fertile window earlier and others much later. Women should be advised that the timing of their fertile window can be highly unpredictable, even if their cycles are usually regular
— The timing of the “fertile window”. AJ Wilcox, D. Dunson and D Baird.

Clinical guidelines state that women typically ovulate between days 10 and 17, however, research has found that this is true for only about 30% of women, even if they have a regular cycle (source). For more than an educated guess based on averages, and to determine your optimal time for intercourse, you may need to use our own biomarkers such as your hormones, temperature and cervical fluid to predict your individual ovulation cycle.

How to Find your Fertile Window:

If you have intercourse every day or every other day until you get pregnant, you will likely not miss your fertile window. For the majority of couples, however, it is not sustainable or even possible to have intercourse this often. Especially if it takes longer than a a couple of months to get pregnant. Here is a table of Advantages and Disadvantages of each method from Timing Intercourse to Achieve Pregnancy: Current Evidence



Fig. 2: BBT Charting Example 

Fig. 2: BBT Charting Example 


  • Track your cervical mucus cycle: 

Cervical secretions, and in particular the changes in cervical secretions, are suggestive of a preovulatory estrogen effect. If you notice when your cervical mucus becomes increasingly more slippery, clear and stretchy, your body is getting ready for ovulation. This method can identify the fertile window 5-7 days before ovulation and the estimated day of ovulation.

The disadvantage is that there tends to be about a month learning curve and daily observations may be inconvenient to some (source). Women who use the Family Planning Method often use cervical mucus tracking in combination with temperature measurements. 


  • Track your temperature:

There are different methods you can use to collect temperature including where (oral, skin, vaginal) and when (continuous or once a day). The once a day readings use basal body temperature (BBT) to determine when ovulation occurred. BBT rises shortly after ovulation and requires dedication to waking up at the same time every morning and charting the results. NOTE: the BBT rise is identified retrospectively, so this method won't be able to tell you when you are fertile but it can give you an idea of when you ovulate.  

Capturing the rise and fall of progesterone to consistently predict the fertile window may require more than oral or skin temperature readings. Continuous core body temperature measurements has been shown scientifically to be able to predict the fertile window (source). Core body temperature is more reliable than oral and skin temperature measurements (source) as it is not impacted by medication, exercise or other external factors. There are vaginal monitors you can purchase to capture your core body temperature readings. 

  • Track Luteinizing Hormone (LH): 

LH is a hormone produced by your pituitary gland. Your body always makes a small amount of LH, but just before you ovulate, you have an LH surge. Products that test for this LH surge, which usually happens 1-1½ days before you ovulate, must be carefully used for accuracy. These products are known as ovulation predictor kits (OPKs).  To track LH, you will need to purchase LH sticks that detect your levels via urine. Typically, you will urinate on the sticks soon after your period ends every morning until you ovulate. 

The disadvantage is that the results can be confusing and this method typically does not work well for women with irregular cycles. Additionally, OPKs identify only a small part of the fertile window and therefore may not identify the days of highest probability of conception


  • Calendar method/Apps/Calculators:

As mentioned in the beginning of the post, you may fall into the 30% of women who can predict ovulation using the calendar calculation. Ovulation occurs approximately 14 days before the onset of menstruation. However, many women do not have such regular cycles and therefore their luteal phase may vary individually from 10 to 16 days, making it harder to predict ovulation by calendar calculations alone.

Using a cycle tracking app may not be the best way to predict your fertile window, however, they are helpful to track your cycles and cycle history.