We do! To work effectively at finding your fertile and non-fertile days, Natural Cycles needs to take sperm survival into account; sperm can survive up to 5 days in the female body. For more reading, please have a look at the original research paper that many related scientific articles are based on.
The fertile window is only six days long if the sperm encounters the right environment in the vagina, uterus, and fallopian tubes. For the sperm to survive longer than a few hours, you need to have fertile cervical mucus. Without that, the sperm will rapidly die, and the fertile window will be reduced to a few hours during ovulation day. 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 5 days waiting to fertilize an egg.
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 first expected ovulation date. This is done by analyzing all your cycles to calculate the first possible ovulation date, in general, more than six fertile days will be given as each cycle may differ from the previous one.