There are a few different types of scans that can be performed during pregnancy, but not all of them can accurately determine how far along you are. An ultrasound scan is the most common type of scan performed during pregnancy, and it can usually give a pretty accurate estimate of your due date. However, an ultrasound scan cannot always tell exactly how many weeks pregnant you are. If your periods are irregular or you ovulate late in your cycle, it may be difficult to know exactly when you conceived. In this case, your doctor may give you a rough estimate based on the size of your fetus and the date of your last period.