There are two main criteria in order for a woman to be able to determine her day of ovulation:
- her menstrual cycle during the last six months lasts for 28 days
- at least two years have passed from menarche
In general, ovulation takes place on the 14th day of a 28-day cycle. Given that the egg is viable for only one or two days, a woman’s fertile period is from the 14th to the 16th day. However, we need to bear in mind that sperm is also able to live for about two days. Therefore, the fertile window in a 28-day cycle is from the 12th to the 16th day.
Nevertheless, due to the fact that there might be a delay in ovulation, we should add two more days, so we have a six day fertile window from the 12th to the 18th day. Those who do not desire a pregnancy and consider fertile days to be “dangerous” should be careful during the second ten-day period of a cycle.
These calculations, as already mentioned, can be done if a woman has regular cycles lasting 28 days. In shorter cycles of 24 or 26 days for example, ovulation happens 2 or 4 days earlier, accordingly. In longer cycles, say 35 days between periods, the fertile window would start seven days later.
If a woman’s menstrual cycle is irregular, then we might fall into error.