The major reasons for delayed and missed periods are stress and pregnancy. Stress over worrying over if you are pregnant is probably the biggest reason a girl your age misses or is late with a period.
The other reasons can be the taking of certain medications such as antibiotics, change in diet or lifestyle or change I exercise routine. There are more reasons though these are the most often reason for a late or missed period other than pregnancy. Also it is not uncommon for teenagers to miss a period while going through puberty as more hormones are released as you go through puberty. [ adviceman49's advice column | Ask adviceman49 A Question ]
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