Your menstrual cycle is your body's way of preparing for a possible pregnancy each month. Part of that is the thickening of your uterine lining, which is where a fertilized egg would implant to begin a pregnancy.If you're not pregnant, your period is how your uterus sheds that extra lining. If you are pregnant, that lining stays put and you don't get your normal flow. This is why a missed period is often the earliest sign of pregnancy.Of course, a delayed or missed period doesn't always mean you're pregnant. If your body is under a lot of stress or you have a hormonal imbalance, you could be experiencing an irregular menstrual cycle.