ivy999 is right. it all depends on your ovulation. in an ideal menstrual cycle, your ovulation days lie within 12-14 days after the first day of your periods. So, if your periods are due this week, then you have long ovulated, say roughly 2 weeks back. This means that your chances of falling pregnant are basically nil. So there is nothing to worry about. And having sex right before and after your periods is usually one of the safest times in accordance to one's menstrual cycle.