What Causes Delay In Periods?

Hello doctor,My name is Revathi.I'm having delay in periods.From last month i'm getting delayed periods.Before marriage also i faced the same problem.After marriage for two months i didn't get the delay in periods monthly menustral was happening But from last month its getting delayed for more than 4 to 10 days.Most of the time i'll be in tension whether i'm pregant.Please help me..