Ovulation and Fertile Window Calculator

How is it calculated?

The menstrual cycle has two phases. In the first, an egg matures, and its length varies from woman to woman. The second runs from ovulation to the next period and lasts about 14 days, however long the cycle is.

The calculation therefore places ovulation 14 days before the next period:

• Day of ovulation: last period + cycle length − 14 days
• Fertile window: begins 5 days before ovulation and ends 1 day after it

The window is that wide because sperm can survive in the female reproductive tract for up to 5 days. The egg itself can be fertilised for about 24 hours after it is released.

Why is it only an estimate?

• The day of ovulation shifts from month to month, even in the same woman.
• Stress, illness, sleep patterns and weight changes all affect the timing.
• In women with irregular cycles the calculation is not reliable.

Physical signs of ovulation

Alongside the calendar calculation, the body gives its own signs:

• A change in discharge: as ovulation approaches, discharge becomes clear, slippery and the consistency of raw egg white.
• Basal body temperature: rises slightly after ovulation. That rise shows ovulation has happened; it does not predict it in advance.
• One-sided pelvic pain: some women feel a brief, mild pain at ovulation.

When should you see a doctor?

• If your cycle is shorter than 21 days or longer than 35
• If cycle length varies markedly from month to month
• If pregnancy has not happened after a year of regular trying under 35, or six months at 35 and over