While blood tests and ultrasound are only reliable methods to confirm that you are pregnant, there are many early pregnancy symptoms that you can sense way before you visit a doctor. Other than a missed menstrual period, there are several other signs that can help you guess. The signs of pregnancy can be divided into presumptive, probable and confirmatory.
Your duration of pregnancy is calculated from your last menstrual period i-e your last menstrual period is considered first week of pregnancy even if you were not really pregnant at that time. Likewise, your date of delivery is calculated using the first day of your last menstrual period. It means that your first few weeks where you did not have any symptoms also contribute while calculating the due date.