From the first day of menstruation to the next menstruation, if MM finds that the length of her menstrual cycle is different, then the longest and shortest days of the cycle can be used as the calculation basis. Let me give you an example:

Note: (In the calculation, constants 18 and 1 1 are obtained through accurate statistical investigation).

Starting from the shortest menstrual cycle (unit: days)

For example,

25 days

Subtract a constant

18 days

The first day of pregnancy is the seventh day.

Then in the longest menstrual cycle,

Such as 3 1 day.

Subtract a constant 1 1 day.

It turns out that the first day of pregnancy is the 20th day.

Then the possible pregnancy date of this MM is from the 7th day to the 20th day of the menstrual cycle, and her safe period is from the 6th day (including the 6th day) and from the 2nd day to the next menstruation!