The biology of the fertile window
The egg survives for just 12-24 hours after ovulation. Sperm can survive in the fallopian tubes for up to 5 days. This means the fertile window spans 6 days: the 5 days before ovulation plus ovulation day itself.
The two days immediately before ovulation and ovulation day itself are when conception is most likely — resulting in pregnancy rates of 20-30% per cycle for couples of average fertility.
How to calculate your ovulation date
Ovulation occurs approximately 14 days before your next period. The formula:
Ovulation day = Cycle length - 14
- 28-day cycle: Ovulation on approximately day 14
- 30-day cycle: Ovulation on approximately day 16
- 35-day cycle: Ovulation on approximately day 21
- 24-day cycle: Ovulation on approximately day 10
Physical signs of ovulation
Cervical mucus: As ovulation approaches, cervical mucus changes to clear, stretchy, and slippery — resembling raw egg white. This directly indicates peak fertility.
Basal Body Temperature (BBT): Your resting temperature rises by 0.2-0.5°C after ovulation due to progesterone. Tracking BBT over multiple cycles can confirm ovulation occurred.
Ovulation Predictor Kits (OPKs): These detect the LH surge that triggers ovulation 12-36 hours before the egg is released. A positive OPK means ovulation is imminent.
What if my periods are irregular?
For women with irregular cycles, physical fertility signs (cervical mucus and OPK testing) are far more reliable than date-based calculation. PCOS is the most common cause of significant cycle irregularity.
The four cycle phases
- Menstrual phase (days 1-5): Uterine lining sheds.
- Follicular phase (days 6-13): Oestrogen rises as follicles develop.
- Ovulation (day 14 in a 28-day cycle): LH surge triggers egg release.
- Luteal phase (days 15-28): Progesterone dominates. PMS symptoms may appear in the final week.