What is sperm survival?
Sperm can survive up to five days in the female reproductive system, so Natural Cycles needs to take sperm survival into account to work effectively at finding your fertile and non-fertile days.
The fertile window is six days long if the sperm encounters the right environment in the vagina, uterus, and fallopian tubes.
Sperm survival (5 days) + Egg survival (1 day) = 6 day fertile window
How can sperm survive in the female reproductive system?
For the sperm to survive longer than a few hours, you need to have fertile cervical mucus. Without that, the sperm will rapidly die. The fertile cervical mucus will help the sperm to travel from the cervix to the uterus and finally to the fallopian tube. Once sperm reaches a fallopian tube, it can live there for up to five days waiting to fertilize an egg.
How does Natural Cycles take sperm survival into account?
Regardless of whether your cervical mucus is fertile or not, Natural Cycles always assumes that your fertile window is six days long, which is the maximum length.
Natural Cycles will therefore give you at least five red (fertile) days before your earliest predicted ovulation date. Natural Cycles calculates the first possible ovulation date by analyzing your historical data.
In general, our cyclers will get more than six red (fertile) days as a safety margin. That’s because each cycle may differ from the previous one.
For more reading, please have a look at the original research paper on sperm survival that many related scientific articles are based on.