I suspect that advice on where to measure waist circumference was developed for men, who don’t necessarily have a well defined waist, particularly if overweight?! And actually possibly for overweight people who can’t tell where their natural waist is?
The following suggests measuring above your belly button:
http://www.health-calc.com/body-composition/waist-to-height-ratio
Most websites I’ve seen suggest that a woman’s waist is her narrowest part.
I’m a pear, and my waist (i.e. my narrowest part and where a waistband sits) is most definitely above my belly button. And nothing anyone says will convince me otherwise 😀