CoupleCare Blog: Calculating fertility days.

Know the most and less fertile days of Menstrual Cycle

How to Calculate Menstrual Cycle Length

Menstrual cycle length is the number of days your menstrual cycle lasts. It is not the same as period length, which is the number of days your period lasts. The day your period starts is considered Cycle Day 1 (the start of a new menstrual cycle.)

The Calendar Method

With the calendar method, you need to keep a record of the length of each menstrual cycle in order to determine when you are fertile. You can use an ordinary calendar. Circle day one of each cycle, which is the first day of your period. Count the total number of days in each cycle. Include the first day when you count. Do this for at least eight cycles (12 is better).

Cycle Record

First Day of Period	Number of Days in Cycle
Jan. 20	29
Feb. 18	29
Mar. 18	28
Apr. 16	29
May 12	26
June 9	28
July 9	30
Aug. 5	27

Find the shortest cycle in your record.

Subtract 18 from the total number of days.

Count that number of days from day one of your current cycle, and mark that day with an X. Include day one when you count.

The day marked X is your first fertile day.

Find the longest cycle in your record.

Subtract 11 days from the total number of days.

Count that number of days from day one of your current cycle, and mark that day with an X. Include day one when you count.

The day marked X is the last fertile day.

Chart Your Calendar Pattern

To predict the first fertile day in your current cycle

To predict the last fertile day in your current cycle

Here is an example:

Predicting your first fertile day. If your shortest cycle is 26 days long, subtract 18 from 26. That leaves 8. If day one was the fourth day of the month, the day you will mark X will be the 11th. That's the first day you're likely to be fertile. So on that day, you should start abstaining from sex or start using a cervical cap, condom, diaphragm, or female condom.

Predicting your last fertile day. If your longest cycle is 30 days, subtract 11 from 30. That leaves 19. If day one was the fourth day of the month, the day you will mark X will be the 22nd. That's the last day you're likely to be fertile during your current cycle. So you may start to have unprotected vaginal intercourse after that day.

As you can see this is a really tricky method but really useful, if you wanna check this in an easy way you can calculate your menstrual cycle right here for free (you just need to leave your email):

http://couplecare.us/calculator/

References:
http://www.plannedparenthood.org/health-topics/birth-control/fam-calendar-method-22139.htm
http://www.whenmybaby.com/menstrual_cycle_length_calculate.php