The day your period starts is considered Cycle Day (CD) 1 — the start of a new menstrual cycle.
The duration of menstrual cycle varies from person to person and is usually between 26 to 31 days. Ovulation occurs approximately in the middle of the cycle, i.e. duration of the cycle – (minus) 14 days.
A week before and a week after the approximate day of ovulation is considered fertile period. The days apart from this are called the safe periods, where the fertilization does not take place.
To calculate menstrual cycle length, you count the number of days from day one of your period (CD 1) up to, and including, the day before the next period starts.
The count does not include the first day of the next menstrual period, because that’s Cycle Day 1 of the next cycle.
Period starts January 3rd, and lasts for 5 days. The next period starts January 27.
MENSTRUAL CYCLE LENGTH CALCULATION:
Count the number of days from January 3rd to January 26th. The counting includes January 3rd (CD 1), and also includes January 26th.
The start day of the next period, January 27, is not included as it starts the new cycle.
ANSWER FOR THIS EXAMPLE: Cycle Length = 24 days
Note: The period length of 5 days does not effect the cycle length calculation. Had this period been 4 days or 6 days, cycle length would still be 24 days.
You have to study your period for a couple of months to know the average number of days your menstrual cycle is.
Every month a woman is in safe period that means if you mate with a woman during that time then the chances of her getting pregnant is almost nil. The chances are almost nil because one has to be very careful calculating the safe period. Any miss calculation will result in fertilization.
You have to note your period for at least 6 months and note the number of days between starting date and the next period to know your safe period. Then identify longest and shortest period cycles.
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