You are fertile for six days per cycle. This fertile window is determined by sperm survival (the five days before ovulation) and ovulation day.
Understanding when you ovulate is key in identifying your fertile window, as you may not always ovulate on the same cycle day. Even if you tend to have regular cycles, your ovulation day may change by several days from one cycle to the next.
The Natural Cycles algorithm rapidly learns each person’s unique cycle patterns in order to detect and predict ovulation. To identify the fertile window, 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 NC° Birth Control, this takes the form of more red days for precaution. NC° Plan Pregnancy users will see fertility on a color scale so they know the days they are most likely to be fertile.
Learn more about how the algorithm detects ovulation here.
How does sperm survival work?
It’s only with the right environment that sperm can survive in the female reproductive system. 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. However, if the cervical mucus isn’t the right consistency, sperm cells can physically be prevented from passing through the cervix, which makes the fertile window shrink.
Learn more about sperm survival here.
What about cervical mucus?
The amount and consistency of cervical mucus changes throughout the menstrual cycle, and is closely linked to fertility. Tracking cervical mucus can be a great way for you to better understand your unique cycle. However, because all our bodies are different, it may be hard to identify clear ovulation patterns. While you can track cervical mucus with Natural Cycles, the algorithm does not use cervical mucus data to calculate your fertility.
Learn more about cervical mucus here.
Is it possible to be fertile after ovulation day?
Compared to sperm, the egg cell has a relatively short lifespan, surviving at 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 during the days before ovulation happens.