The first and most obvious sign for most women is a missed period. Though, some women can have spotting or small amounts of bleeding during the first stage. Other signs of pregnancy may include nausea, constipation, fatigue, and physical changes, such a darkening of the areolas. Your body temperature may elevate slightly, and you may have food cravings. Back pain, constipation, and really, anything out of the ordinary is suspect; especially alongside a missed period.
First weeks of pregnancy and PMS symptoms could be mistaken but missing a period is a good sign you may have a baby growing inside your belly :) Feeling sleepy and getting tired very quick are other signs of early pregnancy. Morning sickness and craving for food you never thought of before could also be pregnancy signs. Get yourself a test and call your doctor for an appointment. Good luck!