1968
The most common cause of DUB in adolescence is anovulation , which is very frequent in the first 2 - 3 post - menarchal years and is associated with immaturity of the hypothalamic - pituitary - ovarian axis .
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Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 None