The biggest sign of pregnancy after unprotected sex is a missed period. However, this missed or late period is not a sure sign as there are many other things like stress, sickness and fatigue that can delay a period besides pregnancy. The best way for you to know if you are pregnant is to do a pregnancy test. These are easy to get nowadays because they are available at drugstores or health centers. It is important to note that the home pregnancy tests are ninety-nine percent accurate if taken after you miss a period.