Pregnancy is technically only possible during the five days before ovulation through to the day of ovulation.
These six days are the ‘fertile window’ in a woman’s cycle, and reflect the lifespan of sperm (5 days) and the lifespan of the ovum (24 hours).
If a woman has sex six or more days before she ovulates, the chance she will get pregnant is virtually zero. If she has sex five days before she ovulates, her probability of pregnancy is about 10%.
The probability of pregnancy rises steadily until the two days before and including the day of ovulation.
At the end of the ‘fertile window’, the probability of pregnancy declines rapidly and by 12-24 hours after she ovulates, a woman is no longer able to get pregnant during that cycle.
This period of her cycle is known as the ‘Safe Period’.
Calculation of Safe Period
The calendar method is most reliable if a woman has a regular period cycle. It is also based on the assumption that ovulation in most ladies takes place at about fourteenth day before the next menstrual flow. The implication of the above is that in a regular period cycle, it can be assumed that ovulation will take place on the fourteenth day before the next period.
If for example, a women’s menstrual cycle varies from 26 days to 31 days cycle.
The shortest cycle [26days] minus 18days = 8th day.
The longest cycle [31days] minus 10days = 21st day.
8th to 21st day of each cycle counting from first day of menstrual period is considered as fertile period. Period other than this fertile period in a menstrual cycle is considered as safe period.
How safe is safe period?
The calendar method for family planning is almost cost-free. All that is required is the calendar and two pens – a red and blue pen. The red pen is used to mark the ovulation day and the preceding ‘unsafe’ six days and the next four days are also underlined in red. The blue pen is used to underline the other days as the ‘safe days’.
The major shortcoming is that the calendar method could be quite tricky, since the pattern of fertility can suddenly change appreciably in the same person over time. Like some ladies would have realized that sometime ago their cycles might have hovered around 28 days and later came down to say 24 days or more – that is stretching to say 33 days. This is common and may be normal.
Safe period is not 100% reliable and so caution should be taken.