Hi Zoe and welcome to the boards. You'll find good support and advice here. What exactly do you mean when you say your Mom has "full-blown osteoporosis"? Has she actually had a fracture or are her T-scores just very low? I'm not familiar with an ankle scan as a measure of T-score, but I can tell you that different bones in the body will score very differently--at least that is the case with me.
Are you a small-boned person? Are you still having menstrual periods? Do you have any systemic illnesses or malabsorption issues? Any of these can influence density.
Before you worry yourself into a frenzy (been there, done that too many times) get yourself a copy of "The Myth of Osteoporosis" by Gillian Sanson. You might also want to request a more conventional DEXA scan of your hips and lumbar area as a comparison. Hope this advice helps; if you have any other questions, ask away and I/others will certainly respond

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