Can We Predict Ovulation Using Daysy?

There are only six days each cycle when pregnancy is possible: the days leading up to ovulation and ovulation day itself. Knowing when your fertile window falls is genuinely useful information, whether you're trying to conceive or simply want to understand your cycle better.

It's a common assumption that ovulation happens on day 14 of a 28-day cycle. In reality, around 70% of women ovulate outside the expected day 13 to 15 range. Cycles vary, ovulation shifts, and a one-size-fits-all approach simply doesn't reflect how most women's bodies actually work.

One of Daysy's features is an ovulation prediction based on your personal cycle history. This is primarily used by women planning a pregnancy, but it also helps with general cycle awareness and life planning.

It's important to understand that the ovulation prediction is completely separate from Daysy's fertility status indicators (red and green days). The accuracy of those indicators is not affected by the prediction.

Here are the key things to know about how Daysy's ovulation prediction works:

1. The ovulation marker (shown as a white star in your current cycle) is a prediction only (see figure A). It is not a confirmation that ovulation has occurred on that day. Because it is a prediction, it can change as your cycle progresses and more data becomes available.
   Important: Ovulation can only be definitively confirmed by ultrasound showing corpus luteum formation, combined with a peak progesterone level detected in the blood during the luteal phase.
   You will also know ovulation has occurred when Daysy begins showing green fertility indicators (see figure B). This switch from red or yellow to green is based on your daily basal body temperature pattern, not on the prediction itself.
2. Once you are 5 days into your next cycle, Daysy reviews your previous cycle in full and updates the ovulation marker from predicted to calculated status if the two differ (see figure C).

The ovulation and menstruation predictions are set at the start of each cycle and remain fixed. Daysy tracks your cycle in real time and updates your daily fertility indicators accordingly, independent of those initial predictions.

Before Ovulation: The Pre-Ovulatory Phase

Daysy's ovulation prediction for the current cycle does not directly determine your pre-ovulatory green days, particularly once you have moved past the initial learning phase. In the early cycles of use, Daysy has limited personal data to work with, so it treats all days after cycle day 5 through to ovulation as potentially fertile. Over time, as more data accumulates, the algorithm identifies infertile days in the pre-ovulatory phase by learning from your menstruation entries and daily BBT readings. The earliest ovulation confirmed by a characteristic temperature rise in your history is the primary reference point for calculating pre-ovulatory green days.

Detecting Ovulation Day

Daysy is designed to identify your full fertile window, not just a single predicted ovulation day. Ovulation is detected retrospectively by identifying a temperature shift toward the end of the cycle, confirming that ovulation has taken place based on your actual BBT data rather than a prediction made at the start of the cycle.

After Ovulation: The Post-Ovulatory Phase

Once the fertile window closes, the infertile post-ovulatory phase begins. After ovulation, the corpus luteum secretes progesterone, which has a thermogenic effect: it causes BBT to rise by approximately 0.2 to 0.3 degrees Celsius. This temperature rise is a reliable, consistent indicator that ovulation has occurred.

Daysy uses this characteristic BBT rise, sustained over a defined period above the pre-ovulatory average, to identify that ovulation has taken place and begin assigning post-ovulatory green days. Each new day, statistical analysis is applied to reassess whether you are still in the elevated-temperature luteal phase. This process is entirely temperature-driven and is not based on the predicted ovulation date set at the beginning of the cycle.

A note from Period Wisdom Boutique

Period Wisdom Boutique exists because we believe women deserve to understand their bodies. Not in a clinical, overwhelming way, but in a way that feels personal, empowering, and genuinely useful in everyday life.

Understanding how Daysy calculates your fertile window is a perfect example of that. When you know what the ovulation marker actually means, what the colour changes are telling you, and how your temperature data drives those indicators, you stop second-guessing and start trusting the process. That knowledge is what we're here to help you build.