Ovulation is the point in your menstrual cycle where one of your ovaries releases an egg. Typically an egg cell survives in the female reproductive system for between 12 and 24 hours; this varies from cycle to cycle. Natural Cycles works by detecting ovulation; this can be identified by increased basal body temperature caused by hormones before ovulation day.

Basal body temperature is the lowest resting body temperature measured to 2 decimal points. Read more about how Natural Cycles works here. In biological terms, your ovulation day is the only day when you can get pregnant, as this is the point when the sperm and the egg actually meet. However, it’s really important to know that you are fertile before this! It’s not enough to avoid unprotected sex on your ovulation day as there is still a risk of pregnancy; this is due to sperm survival.

Sperm survival varies due to many factors. The maximum time sperm can live in a woman’s reproductive system is five days. Some sperm will live for a shorter time, depending on the conditions in the vagina, fallopian tubes, and uterus. (Such as the consistency of cervical mucus.) Without the right fertile conditions, sperm will die within a few hours.

Sperm can live for up to 5 days in the right conditions. With this in mind, the fertile window can be calculated to be six days long; this includes the five days sperm can survive, plus the one day of ovulation.