Understanding Your Fertile Window
The fertile window is the period during your menstrual cycle when pregnancy is possible. It spans approximately six days — the five days before ovulation and the day of ovulation itself. This is because sperm can survive in the female reproductive tract for up to five days, while an egg can only be fertilised for 12–24 hours after ovulation.
How Ovulation Is Calculated
For a standard 28-day cycle, ovulation typically occurs on day 14 — counted from the first day of your last period. For longer or shorter cycles, ovulation is estimated to occur approximately 14 days before the start of your next period. This is why the formula is: cycle length minus 14 days equals the ovulation day.
Signs of Ovulation
Physical signs that ovulation is approaching or occurring include a change in cervical mucus (becoming clear and stretchy, similar to raw egg white), a slight rise in basal body temperature (typically 0.2–0.5°C), mild one-sided pelvic pain (known as Mittelschmerz), and increased libido. Ovulation predictor kits (OPKs) detect the LH surge that occurs 24–36 hours before ovulation and are widely available from UK pharmacies.
Is this calculator accurate?
This calculator provides an estimate based on average cycle patterns. Cycle length can vary month to month, and factors such as stress, illness, and hormonal changes can affect ovulation timing. For the most accurate results, track your cycle over several months and consider using ovulation predictor kits.
Can I get pregnant outside the fertile window?
Pregnancy is theoretically possible only during the fertile window (the 6 days ending on ovulation day). However, because cycles can be irregular, it is difficult to predict the fertile window with certainty without tracking.
What if my cycle is irregular?
If your cycle length varies significantly from month to month, this calculator will be less accurate. Speak to your GP if you have concerns about irregular cycles, as this can sometimes indicate underlying hormonal conditions.