What Causes Menstruation Delay?

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Hi ,, generally my period date is 15,,, wht 2 say its a improper period dates . like irrgular somestimes first week or some time middle of the week .. also i consult with my doctor friend about this and she suggest me to take Evecare tonic ..I have taken it .. but now whole month is finsih now and my period is not comes .. Could you pls help me on this ?