Family Planning Plus

It really depends on the length of your menstrual cycle. If you normally have cycles longer than 27 days, pregnancy is unlikely on day 7. You can use the calendar method to predict your most fertile days. However, the calendar method is not an exact science and should only be used if your periods are regular and your cycles normally last 27 days or longer.

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 (from day one to the day before your next period). Do this for at least eight cycles (12 is better). 

To predict the first fertile day in your current cycle:

• 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.

 To predict the last fertile day in your current cycle:

• 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.

 Here is an example:

To predict your first fertile day:

If your shortest cycle is 28 days long, subtract 18 from 28. That leaves 10. If day one (your first day of bleeding) was February 6, the day you will mark X will be February 15th. That’s the first day you’re likely to be fertile. So on that day, you should start abstaining from sex or start using birth control.

To predict your last fertile day:

If your longest cycle is 32 days, subtract 11 from 32. That leaves 21. If day one was February 6, the day you will mark X will be February 26. 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.

In this example, February 15th through the 26th are unsafe days. All the others are safe days.

Source: http://www.plannedparenthood.org/health-topics/birth-control/fam-calendar-method-22139.htm