"Springing"

[cmf1]

Just thinking about the origin of the term used to describe "close to calving".
The udder getting "springy"?
The vulva getting "springy"?
The ligaments getting "springy"?
All the above?
None of the above?
Time of year?
I've always thought I knew what it meant, just curious if there's a more exact meaning and why "springing"?

 

[jerry27150]

the udder starting to fill

 

[upfrombottom]

Usually means a sudden change, as in springing into action.

 

[Lucky_P]

The first three.