Calculating Ovulation: How to Determine Your Fertile Days

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written by Zappelphilipp Marx•6 June 2025
Illustration of an egg shortly before ovulation

An egg cell is only fertile for a few hours each cycle—this brief window determines whether a pregnancy can occur. In this guide, you’ll learn how your cycle unfolds, which methods reliably detect ovulation, and which mistakes to avoid.

Understanding Cycle Phases and Hormones

  • Menstruation (Day 1–5): The lining of the womb is shed, and oestrogen and progesterone are at their lowest.
  • Follicular Phase (Day 1 until Ovulation): FSH stimulates a follicle to mature; rising oestrogen rebuilds the lining.
  • Ovulation (usually Day 12–16): An LH surge releases the mature egg. It remains fertile for about 12–24 hours.
  • Luteal Phase (about 14 days): Progesterone from the corpus luteum maintains the lining. If fertilisation doesn’t occur, progesterone drops and a new cycle begins.
Graph of FSH, LH, oestrogen, and progesterone throughout the menstrual cycle
Hormone patterns and cycle phases at a glances

Calculating Fertile Days

Sperm can survive up to five days, while an egg is only fertile for about 24 hours. Therefore, the highest chance of conception is during the six days leading up to and including ovulation1.

  • Knaus-Ogino Method: First fertile day = shortest cycle − 18; last fertile day = longest cycle − 11.
  • Practical Tip: Having intercourse every two to three days covers the entire window without added stress.

Cycle Tracking and Ovulation Tests

  • Calendar Apps: Useful for regular cycles, but only provide rough estimates.
  • Basal Body Temperature: Take your temperature daily first thing after waking. A rise of 0.2–0.5 °C indicates ovulation retrospectively.
  • Cervical Mucus: Clear, stretchy mucus signals peak fertility.
  • LH Tests: Detect the LH surge 24–36 hours before ovulation—ideal for irregular cycles.
  • Wearables: Sensors measure temperature and heart rate variability; with AI, they achieve around 90 % prediction accuracy2.

How to Use Ovulation Tests Correctly

  1. Start five days before your earliest expected ovulation.
  2. Use second-morning urine—concentrated but not overly retained.
  3. Hold the test strip in the urine stream for ten seconds or dip it into collected urine; read the result per the manufacturer’s instructions.
  4. Plan intercourse on the day of the test and again the next day.

Recognising Ovulation Symptoms

  • Glassy, stretchy cervical mucus
  • Mild pulling pain in the lower abdomen (mittelschmerz)
  • Cervix rises higher and feels softer
  • Basal body temperature rises the next day

Ovulation Not Occurring? Causes and Treatments

Common causes include PCOS, thyroid disorders, or luteal phase defects. Follow these steps:

  1. Keep a cycle log and have your hormone levels (including thyroid) checked.
  2. Normalise your weight and reduce stress.
  3. Consider treatment with clomifene or letrozole to stimulate ovulation, if needed.
  4. Consult a fertility clinic for IUI or IVF if other measures aren’t sufficient.

Myths and Facts About Ovulation

  • Myth – Ovulation always occurs on day 14 of the cycle.
    Fact: Ovulation timing varies depending on the length of your follicular phase. Even in a 28-day cycle, it can occur between day 10 and 17; if your cycle length differs, the window shifts accordingly.

  • Myth – If you don’t feel mittelschmerz, you’re not ovulating.
    Fact: Only about one third of women feel abdominal pulling. The absence of this pain doesn’t mean ovulation didn’t occur.

  • Myth – Daily sex significantly increases the chance of pregnancy.
    Fact: Having sex every two to three days is sufficient. It maintains sperm quality while covering the entire fertile window.

  • Myth – Basal body temperature can predict ovulation.
    Fact: A temperature rise indicates that ovulation has already happened. Predictive markers like LH tests or cervical mucus observation are better for planning.

  • Myth – Stress has no impact on fertility.
    Fact: Chronically elevated cortisol can delay or suppress the LH surge. Regular relaxation exercises have been shown to improve pregnancy chances.

Sources & Further Reading

  1. Wilcox AJ et al. Timing of Sexual Intercourse in Relation to Ovulation. N Engl J Med, 1995.
  2. Shilaih M et al. Wearable Sensors Reveal Menses-Driven Changes in Physiology. J Med Internet Res, 2019.
  3. van der Velden J et al. Innovative Approaches to Fertility Tracking, 2023.
  4. World Health Organization. Infertility – Fact Sheet, 2024.

Conclusion

A combination of a calendar app, LH test, and basal body temperature offers the most reliable way to find your fertile window. Pay attention to your body’s signals and record your cycle. If efforts don’t lead to results, it’s wise to seek medical evaluation promptly—modern reproductive medicine has solutions for almost every obstacle.

Frequently Asked Questions (FAQ)

You can narrow down ovulation more precisely by using an ovulation test (LH test) three to five days before your expected ovulation and also measuring your basal body temperature daily. This way, you detect both the preceding LH surge and the subsequent temperature rise.

For irregular cycles, it’s best to use ovulation tests and observe your cervical mucus. Track several cycles to determine averages, and begin testing five days before your earliest expected ovulation until a positive LH test appears.

The Knaus-Ogino method calculates the first fertile day as “shortest cycle − 18” and the last as “longest cycle − 11.” It provides a rough estimate but is only reliable for fairly consistent cycles and should ideally be supplemented with LH tests or cervical mucus observation.

Begin ovulation tests five days before your earliest expected ovulation. For example, if your shortest cycle is 28 days with ovulation around day 14, start testing on day 9 and continue daily until the test is positive.

The LH test measures the surge of luteinising hormone (LH) in urine. A positive test signals that the LH peak has been reached and ovulation is likely to occur within 24 to 36 hours.

Basal body temperature confirms ovulation retrospectively, as the temperature only rises half to a whole day after the egg is released. For accurate prediction, you should combine it with LH tests or cervical mucus monitoring.

To observe cervical mucus, insert a finger into the vagina before using the restroom and examine the mucus’s consistency. Just before ovulation, it’s clear, thin, and very stretchy—similar to raw egg white.

The best time is during the five days before ovulation and on the day of ovulation itself. Sperm can live up to five days and the egg only 12 to 24 hours, so you achieve the highest chance of conception by having intercourse within this window.

Yes. Ovulation timing is not fixed and varies individually. In a 28-day cycle, it can occur between day 10 and 17. If your cycle is shorter, ovulation can happen correspondingly earlier.

A missed ovulation usually shows up as no LH surge and no temperature rise. A cycle without a temperature increase in the luteal phase or consistently thin, non-stretchy cervical mucus indicates anovulation. In such cases, a hormone test at a gynaecologist is advisable.

High stress elevates cortisol levels, which can weaken or delay the LH surge. Regular relaxation techniques such as yoga, meditation, or breathing exercises help to stabilise hormones and trigger timely ovulation.

A balanced diet with sufficient vitamins (especially vitamin D, B vitamins), minerals (zinc, selenium), and omega-3 fatty acids supports your hormone balance. Antioxidants from fruit and vegetables protect eggs and can promote cycle regulation.

Excess weight can lead to elevated insulin and oestrogen levels, which can weaken the LH surge and cause irregular or missing ovulation. A moderate weight loss can improve cycle regularity and egg quality.

For most women, ovulation occurs in the first cycle after stopping the pill. About 80 % ovulate within six weeks. However, hormonal imbalances like PCOS may delay ovulation.

Cycle apps use algorithms to generate predictions based on your entered data. They collect information on period length, symptoms, and vital signs. Combined with basal body temperature and LH tests, they can be very effective for predicting ovulation.

Early ovulation may show up as an early positive LH test, unusually early clear cervical mucus, or a temperature rise in the second half of the cycle. Pay special attention if you have short cycles and watch for these signs right after your period.

Measure your basal body temperature every morning at the same time right after waking, before getting out of bed. A temperature rise of 0.2–0.5 °C indicates the past ovulation.

Yes, medications like clomifene and letrozole stimulate ovulation by affecting hormone production. They are often used for women with irregular ovulation. Dosage and duration are determined by a physician.

Polycystic ovary syndrome (PCOS) often leads to irregular or absent ovulation. Weight loss, lifestyle changes, and medication (e.g., metformin, clomifene) can help normalise the cycle and promote regular ovulation.

Visiting a fertility clinic is recommended if you’re under 35 and have been trying unsuccessfully for a year, or if you’re over 35 and haven’t conceived after six months. With known conditions like endometriosis or thyroid disorders, you should seek evaluation earlier.