? for women out there w/a low BF%!

For years I have bounced between 17-19% BF and I can't seem to go any lower.

Women with a 2/4/6 pack...what am I missing?

I work out hard! -body weight, weights, whole body, primal movement, yoga...and I eat clean Whole Foods (super clean could be a missing link. But Do I really need to micromanage to get there? ), and cardio I do struggle with becauSe it causes neck/back pain from the scoliosis. I still get in 3-4 sessions of HIIT per week and I just drop to that lower BF% and no lower. I'd love to figure out the trick before conceiving. ;)