I have some that look like they are about to pop for three weeks. Others just go off to the brush and calve. Milk comes up overnight it seems.
They can drive you nuts. If you see one rolling on the ground, she's in trouble. If you see feet coming out pointed upward, she's in trouble. Other than that, you can pretty much leave them alone. Check to see that she cleans up her calf and lets it get colostrum.
Heifers tend to lay down, get up and walk with the herd, and lay down again when they are going in to labor. Over and over. Others don't do anything like this.
Welcome to the cattle industry. Calves are the second best thing about this business - second to getting that pay check when the calves get sold.