[header] How to determine why your period is late [title] Consider the possibility of pregnancy. [step] One of the most well-known causes of a late period is pregnancy. When you become pregnant, you no longer shed the lining of your uterus, which is what causes your period each month. 
[substeps] If you are sexually active, you should consider pregnancy as a possibility-even if you are careful. No form of birth control is 100% effective, so there is always a chance.