If a women has a regular menstrual cycle of 28 days, lasting for 4-5 days, it is believed that sex - even without a contraceptive - between days 5 to 7 and days 21 to 28 will be safe from the risk of pregnancy. (Count the day when the bleeding starts as day number 1 of the cycle). However, please note that this method is not 100% foolproof and occasionally can result in pregnancy.