Your waist is a fixed point on your body, because it relates to your bone structure.

Your navel is soft tissue and its position on your body is highly variable. For a start, it may or may not be on your body's centre line!

Twins, stood side by side will not necessarily have their navels in the same place. How they were lying in the womb affects the final position even before they start to grow.

If a pathologist is given one of long bones of a human being they will be able to tell you, with a high degree of accuracy, the height of the individual. There are very good and accurate tables for this. On the other hand, given any number of bones (up to and including a full skeleton), they would not be able to place the navel with any more accuracy than you or I.

As a male ages, he is likely to put on a 'gut'. That means the skin has to stretch to accommodate the extra fat layer. Gravity will drag it downwards. If he looses the gut, there is no guarantee that the skin will shrink back equally above and below the navel.

It is my belief that the older we get, the more our navel moves south.

Regards

Chas