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The best thing to do would be get an urine pregnancy
test now , since it has been slightly over a month since the last period. If positive, do consult an obstetrician. If not, then stress
could be the reason for missed periods. Stress can cause hormonal
imbalance, which in turn can suspend her periods, temporarily. Once the causative factor for stress has been removed, her periods will come back to normal.
However, if her pregnancy test continues to remain negative and she is not under any stress, then do consult a doctor, who may advise an ultrasound
scan to identify the causative factor. Also, if she is not pregnant, then advise her to eat right and get optimum exercise, to maintain a body weight within normal range.