how will i know if i am safe when i use the calendar method, because based on my experience i let my partner withdraw his sperm inside me after my menstruation, last day of my menstruation.is there a possibility that i might get pregnant?

The calendar method, or rhythm method, is based on the idea that you can calculate when you are fertile based on the assumption that your cycle is exactly 28 days long and you ovulate on day 14, and sperm live for 2 days in the body. While this is true on average for most women most of the time, it's not always true, and thus the calendar method has been found to be only somewhat effective in preventing pregnancy as compared to other methods of birth control. So, you are determining based on the assumptions that you are not fertile during the time after your period, and so sex will not result in pregnancy. This is a wrong assumption, because you may ovulate earlier than you expect. Thus, you should not consider this a safe method of pregnancy prevention.

There are several newer natural methods of birth control you can try that are more effective and easier than the calendar-rhythm method. Consider the Standard Days Method (Cycle Beads) or the TwoDay Method. You can learn about both of these at this site.

Best of luck,
Dr. Williams