Calculating Ovulation: How to Determine Your Fertile Days

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.

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.