If you want to know your waist when you're trying to find your size for clothes/sewing etc then it's the narrowest part of your waist that's roughly 1-2". You can also find it by bending to the right (or left, doesn't matter) and measuring where your waist bends. For me it varies between 33-34" depending on if I'm bloated or not.
Now, if you measure at your navel or below you and you're at all prone to bloating (especially during your TOM) then you're going to see A LOT more variation. For instance, around my built-in muffin top I can be as small as 37" or up to 40/41" if I've had a huge meal or it's TOM or I'm just bloated in general. Your navel can also have many more variations so I'd be pretty surprised if that's where they're measuring for the study because you'd have a lot more factors going into besides just how much fat a person is storing around their waist. Did they check if the woman had just ate? Is she on her period? etc
I do think measuring around your natural waist vs. your lower belly probably also shows more of a difference between visceral and subcutaneous fat. Even thought I don't have a "slim" natural waist yet, I don't have a lot of fat that I can just pinch around there (so it's probably stored more around my internal organs

). However, on my lower belly I can grab a huge chunk of fat...
I totally agree about the inch waist thing not making a ton of sense when it comes to one'S height. I read somewhere that ideally you divide your height in inches in half and your waist should be at or below that number. That seems to make more sense to me and for a 5'6" woman that would mean 33" or below. I'm right at the line between obese and overweight so it happens to coincide with that milestone in my BMI too. I've often wondered for other women where those too coincide...
As for Waist-to-Hip ratio. CDC lists being under 0.8 as "safe", although I've heard 0.7 is better and that 0.6 or under is what you see in some beauty icons. Right now I'm under 0.8 and even when I was in the middle of my BMI and VERY athletic I never was at 0.7 or under so I have a hard time believing that's going to happen. Some women are just not going to carry enough weight around their hips to pull that off...
Sorry for the rambling but I've watched my inches closely because my body doesn't like to give up pounds as easily so this is a subject near and dear to my heart.
