Ovulation Tracking Methods From Prediction to Calculation Explained

Many women use ovulation tests to determine ovulation. With this test, you measure your urine using a test stick, which shows you how much luteinizing hormone is present in your urine.

However, these tests are not always reliable, as they may not work equally well for every woman and lifestyle. Errors can occur, especially if your urine is very diluted or if you test first thing in the morning.

A better alternative is to measure and observe your basal temperature.

If you want to get pregnant, of course, you want to know when your body is ready. This is where ovulation prediction comes in. There are only six days in each cycle when it’s possible to get pregnant – this time frame is called the fertile window.

Understanding the fertile window

The fertile window is determined by the maximum lifespan of sperm (5 days) and the viability of the egg (about 24 hours). This means that even if you have intercourse today, sperm could still meet a fertilizable egg up to five days later.

Maximizing Your Chances of Conception

Knowing this timeframe precisely not only significantly increases your chances of pregnancy, but it also allows for stress-free planning – without the guessing game.

The 6 Fertile Days – A Biological Window

Sperm Lifespan

• Sperm can survive in your body for up to 5 days under optimal conditions.

Egg Lifespan

• The egg, on the other hand, is in a hurry: it only lives for around 12–24 hours.

Why ovulation doesn’t always happen on day 14*

Cycle Lengths Vary

• Not every woman has a 28-day cycle – many have cycles of 24, 32, or even 40 days. Ovulation shifts accordingly.

Statistical Facts and Variations

• Studies show that only 30% of women ovulate between days 13–15. The rest deviate – often significantly.

Basal Temperature as the key to prediction

Before ovulation, your basal temperature is slightly lower. After ovulation, it rises noticeably due to the hormone progesterone.

Visualization: The white star indicator

The small white star indicates the predicted ovulation – based on data from your previous cycles (see Figure A). This prediction is based on everything Daysy knows about you so far. It is NOT a confirmation that ovulation will definitely occur on that day. Since it is merely a prediction, it is subject to change based on the variability of your cycles.

Important to know

The ovulation and menstruation predictions are statistically set at the beginning of the cycle. However, by measuring your basal temperature each morning, Daysy tracks your cycles in real-time and provides highly accurate fertility indicators every day, regardless of the initial predictions.

Daysy was designed not only to identify and determine the predicted day of ovulation for the user but also to identify the full potentially fertile window. Due to the characteristic rise in basal temperature, it can be accurately (retrospectively) determined at the end of the cycle that ovulation actually took place.

Once you’ve entered a few days into your next cycle, Daysy evaluates your previous cycle in its entirety and shifts the marker (white dot) from the predicted ovulation to the calculated ovulation if it differs (see Figure C).

The transition from red to green days is not based on the ovulation prediction, but on the daily measurement of your basal temperature.

The ovulation prediction, however, is completely independent of the display of fertile (red) and non-fertile (green) days. The transition from red to green days is not based on the ovulation prediction, but rather on the daily measurement of your basal temperature. (see Figure B)

Pre-Ovulation Phase (driven by previous cycles)

Daysy’s ovulation prediction for the current cycle does not necessarily impact the pre-ovulatory green days, particularly after you have gone through the learning phase. When you first begin using Daysy, due to the lack of personal data in the early cycles of use, the algorithm assumes that all days after cycle day 5 and through ovulation could be fertile. This is adjusted by Daysy over time with more available data, in which infertile days during the pre-ovulation phase are calculated by learning from previously entered data and daily basal body temperature (BBT). Your earliest recorded ovulation confirmed by the corresponding characteristic elevated basal temperature is mainly responsible for your pre-ovulatory green days.

Post-Ovulatory Phase (Temperature-driven)

After the fertile (red/yellow) days, the infertile post-ovulatory phase begins. Immediately after ovulation, the hormone progesterone is secreted by the corpus luteum. Progesterone has a thermogenic effect which causes the BBT to rise by approx. 0.2-0.3°C after ovulation. Due to the predominant secretion of the corpus luteum, progesterone levels and the associated increase in BBT after ovulation is a very constant retrospective indicator that ovulation has occurred. The status covering the post-ovulatory phase is temperature-driven and also not based on the predicted ovulation set at the beginning of the cycle. After ovulation is identified by a characteristic rise of BBT for a defined period of time over the average BBT of the pre-ovulatory phase, Daysy starts to assign post-ovulatory, green infertile days to the days thereafter. On each new day, statistical analysis is used to re-evaluate whether you are still in the high-temperature infertile luteal phase.

With this feedback loop of daily data and statistical validation, it is possible to provide a scientifically proven analysis in the identification of your personal fertile window.