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There are several reason of missed or delayed period.
First exclude pregnancy by doing blood test
for hcg or confirm by USG lower abdomen.
Common reasons are PCOD,over wt,hormonal imbalance,irregular eating habit,thyroid disorder
,stress, psychological upset etc.
Need to investigate properly to rule out definite pathology. Do Thyroid profile,PPBS, LH/FSH etc and consult with your gynecologist.
Control over wt with regular exercise and it increase fertility
rate.Avoid stress by regular meditation
. Take healthy diet and proper sleep.