In this instance the issue is no longer height/weight but % of body fat, which looks here to be under 5%.
“Studies indicate that the body’s fat content must account for 17% of the body’s weight before menarche can occur and that, at age 18 years, the fat content must be at least 22% for the maintenance of regular menstrual cycles.”
This is not that at all. She is a bit under “medically ideal” weight, but nothing like that. Significantly healthier than most of us.
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In this instance the issue is no longer height/weight but % of body fat, which looks here to be under 5%.
“Studies indicate that the body’s fat content must account for 17% of the body’s weight before menarche can occur and that, at age 18 years, the fat content must be at least 22% for the maintenance of regular menstrual cycles.”
https://pubmed.ncbi.nlm.nih.gov/4053451/