Your fertile days / the fertile window is the 6 day period in which a woman can get pregnant from having sexual intercourse. The fertile window is determined by the sperm survival of 5 days and the 24h during which the egg is released and dies.
After ovulation, the egg can maximally survive up to 24h. Studies have shown that the quality of the egg deteriorates very quickly; causing the probability of conception to decrease rapidly every hour once the egg has been released. Therefore, for optimal chances of conception, sperm should already be present in the fallopian tube before the egg is released, which means the most fertile day of the woman’s cycle is rather the day prior to ovulation than the day of ovulation itself.
Natural Cycles knows that your fertile window is 6 days long – the maximum length regardless of the regularity of your cycle. The algorithm accurately calculates the variation of the ovulation day for each woman to ensure that the fertile window is covered with red days. This is why you in most cycles get more than 6 days, as ovulation rarely occurs on the same day in each cycle the algorithm gives a couple extra days as a precaution.
The fertile window is only six days long if the sperm encounters the right environment in the vagina, uterus, and fallopian tubes. In order for the sperm to survive longer than a few hours, you need to have a 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 the 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 6 days long.