While you’re only fertile for 6 days in any cycle, Natural Cycles will always allocate a few extra days for extra precaution. Read on to discover what defines the fertile window and how our algorithm accounts for shifting fertile days.
This fertile window is determined by sperm survival (5 days) and ovulation day. Compared to sperm, the egg cell has a relatively short lifespan, surviving at the most 24 hours after ovulation. The quality of the egg deteriorates very quickly once it is released, meaning the chances of conception decrease every hour after ovulation. For those trying to conceive, the best chance of fertilizing the egg cell is by ensuring sperm are already in the fallopian tubes by having sex the day before ovulation happens.
Natural Cycles uses a safety margin when calculating the fertile window, allocating extra fertile days to take into account the variation of ovulation day. For those using the app as birth control, this takes the form of more red days for precaution. Plan users will see fertility on a color scale so they know the days they are most likely to be fertile.
It’s only with the right environment that the sperm can survive in the female reproductive system. Sperm requires cervical mucus to help it travel towards the egg cell. With the right consistency of cervical mucus, sperm can easily travel to the fallopian tubes where it can survive for days before fertilizing the egg cell. If cervical mucus isn’t the right consistency, sperm cells can rapidly die off and the fertile window shrinks.
While you can track cervical mucus with Natural Cycles, the algorithm does not rely on cervical mucus data to calculate your fertility. Instead, the algorithm uses the maximum six-day fertile window and adds extra days to account for the natural changes that happen in each cycle.