Ovulation

The egg travels without a timetable

The previous article “Fertile Days” was about the big meeting of sperm and egg.

But before that can happen, the egg first has to set off.

The problem with its journey? There is no reliable timetable. And sometimes the departure is even cancelled altogether.

Egg-Advisor as a train conductor on the platform with the egg, a suitcase and a departure board

EggCademy · Bonus knowledge BioShock – Open at your own risk Warning: contains follicles, hormones and an unexpected amount of biology.

Phase 1: The egg gets ready to travel

The menstrual cycle can be roughly divided into two phases: the time before ovulation and the time after it.

Egg-Advisor watches ovulation: a mature follicle opens in the ovary and releases the glowing egg — The moment of ovulation: the mature egg is released into the fallopian tube

In the ovary, an egg matures in a small fluid-filled sac – the so-called follicle. It is getting ready to travel, so to speak. That is why this first section is called the follicular phase.

Hormones control this preparation. How long it takes can vary from cycle to cycle.

Once the egg is ready, a hormone signal triggers ovulation: the mature follicle opens and releases the egg. It is then taken up by the fallopian tube.

Ovulation has happened. The journey begins.

Phase 2: The egg is on its way

After ovulation, the second phase of the cycle begins – the so-called luteal phase. The follicle left behind turns into the corpus luteum, which mainly produces the hormone progesterone. It helps to prepare and maintain the uterine lining for a possible implantation.

Egg-Advisor in the glowing fallopian tube: the egg travels toward the uterus — On the way through the fallopian tube: the egg’s journey toward the uterus

The egg travels through the fallopian tube and can only be fertilized for about 12–24 hours along the way.

If it is fertilized, an embryo can develop from it, which moves into the freshly prepared guest room – the uterine lining – a few days later.

If there is no pregnancy, the corpus luteum breaks down. The progesterone level drops and the next period begins.

The luteal phase begins after ovulation and ends when the next period starts. Its length is not fixed: 11 to 17 days are considered the usual range.

The myth of day 14

Even without a reliable timetable, ovulation can at least be estimated.

A common rule of thumb is: cycle length minus 14 days. For a 28-day cycle, that gives day 14.

Egg-Tracker calculates slightly differently: it places ovulation 14 days before the expected start of the next period. In our example, that gives day 15.

Cycle ribbon in Egg-Tracker (dark theme EggClipse), 28-day cycle: fertile days 10 to 16, calculated ovulation on day 15 — **Calculated ovulation and fertile days** Calculated ovulation on day 15 of a 28-day cycle

Cycle ribbon in Egg-Tracker (light theme EggSposure), 28-day cycle: fertile days 10 to 16, calculated ovulation on day 15 — **Calculated ovulation and fertile days** Calculated ovulation on day 15 of a 28-day cycle

How reliable is this rule of thumb?

Johnson and colleagues (2018) studied the timing of ovulation using daily hormone measurements in 949 women.

The surprising result for cycles that actually lasted 28 days:

Frequency of ovulation in 28-day cycles (Johnson et al. 2018)
Cycle day	Ovulation frequency	Note
Day 14	14%	Rule of thumb
Day 15	19%	Egg-Tracker calculation
Day 16	21%	Most common ovulation day in the study
Day 17	17%	Easy to remember

So even with the same cycle length, there is no fixed ovulation day.

What does this mean for Egg-Tracker?

Overall, ovulation days between day 11 and day 20 were observed.

And as we already know: if day 0 shifts, the fertile window moves with it.

If we take these fluctuations into account, the possible fertile days could look like this even for a 28-day cycle.

Cycle ribbon (dark theme EggClipse) with the possible ovulation days according to the study: day 11 to day 20 marked — **Possible ovulation days according to the study** Days 11 to 20

Cycle ribbon (light theme EggSposure) with the possible ovulation days according to the study: day 11 to day 20 marked — **Possible ovulation days according to the study** Days 11 to 20

But even that would be no guarantee. The cycle length itself can also turn out shorter or longer in the current cycle and shift the ovulation window even further forward or back.

And that is exactly why Egg-Tracker is not suitable for contraception.

Can ovulation fail to happen?

Yes, there are cycles without ovulation.

In the Johnson study, no corresponding rise in hormones could be detected in about one in ten participants. However, this does not necessarily mean that no ovulation actually took place.

So what is the bottom line?

The period marks the start and the end of a cycle.
The fertile days lie around ovulation (day 0).
Ovulation has no fixed date. If it shifts, the fertile window moves with it.

Scientific background

Can apps and calendar methods predict ovulation with accuracy?Johnson S et al. Curr Med Res Opin. 2018. PMID: 29749274. Two of the authors were employed by the test manufacturer SPD Development Company.
Clinical investigation of the menstrual cycle. I. Diagram of the normal menstrual cycleJolivet A, Gautray JP. Fertil Steril. 1978. PMID: 620841. The study examined 46 normal menstrual cycles and found a mean luteal phase of 14.26 days, with statistical tolerance limits of 11 to 17 days.
Normal variation in the length of the luteal phase of the menstrual cycle: identification of the short luteal phaseLenton EA et al. Br J Obstet Gynaecol. 1984. PMID: 6743610.
The timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective studyWilcox AJ et al. BMJ. 2000. PMID: 11082086.

The contents are for general educational purposes and do not replace medical advice. Egg-Tracker displays estimated cycle phases, including a fertile window. This information is not a medical diagnosis and not a recommendation, and in particular must not be used for contraception (Disclaimer).