To calculate the fertile period one should always have it in mind that ovulation occurs in the middle of the cycle, around the 14th day of a regular cycle of 28 days.
To identify the fertile period, a woman with a regular cycle of 28 days should count 14 days from the date of the beginning of the last menstruation, because ovulation will take place 3 days before and 3 days after that date, which is when is considered to be the fertile period of the woman.
How to calculate the fertile period in an irregular cycle
Calculating the fertile period in the irregular cycle is not safe for those who are trying to get pregnant or for those who do not want to get pregnant. This is because menstruation does not always occur in the same period, hence the accounts may be wrong.
However, one way to know when is the fertile period in case of an irregular cycle is to write down duration of each menstrual cycle for a year and then subtract 18 days to the shortest cycle and 11 days to the longer cycle.
For example: If your shortest cycle was 22 days and the longest cycle was 28 days, then: 22 – 18 = 4 and 28 – 11 = 17, therefore the fertile period will be between the 4th and 17th days of the cycle.
A more rigorous way of knowing the fertile period in case of an irregular cycle for women who wish to become pregnant is to resort to the ovulation test that is bought in the pharmacy and then look out for the signs of fertile period, such as discharge which is similar to egg white.
For women who do not wish to become pregnant, this is not an effective method and therefore it is important to use safe contraceptive methods, such as condoms or the contraceptive pill.