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The chance of getting pregnant is approximately 14th day of menstrual cycle, 1st day being the first day of bleeding.
First week from starting of period and last week before next period is called safe period when chance of getting pregnant is extremely low.
Usually chance of pregnancy
is from 12th day to 18th day of period. Avoid these days for intercourse.
Another method which is not very reliable, is withdrawing the penis just before ejaculation, so that ejaculation occurs out side the vagina.
Best is to have intercourse during safe period.