Does bmi need to be below 18.5 to cause amenhorrea? or is it enough to be at the low end of the healthy bmi range?

Its a range: There is interindividual variation in the menstrual cycle of women with low body weight. According to the WHO, a BMI of less than 18.5 kg/m2 is considered abnormally low weight for adults. Body weight below 85% of ideal body weight is one of the diagnostic criteria for anorexia. Amenorrhea is often a sign of a health problem rather than a disease itself & it has many causes.