It is a tough call, you leave her out there and she has problems then you are dealing with it in the dark in the field or trying to get her up and in OR bring her in and hope that she settles in quickly and resumes calving and not destroying the corral or barn. Either way a live calf is the desired result.
Personally we try to leave our girls out to choose their "spot" and do their thing without interference. Main variables include the weather and if we have a pretty good idea that the calf is large, i.e. big feet showing and the time she has been working without progress. If we do have to bring them in often we try to wait until they can't "put it off". And we bring some friends for them. It makes a difference how your layout is and how easy it is to get them into the barn, their attitude/wildness and if they have been penned up before. The walking, tail switching, up and down movements all help the cow/heifer get the calf in position and moving properly. Restricting this, adding the stress of humans and the interruption of the general process, I believe, can really slow things down to the point where we pull the calf, possibly before she is truly ready to have it, just to make sure we have a live one (though I have to admit I'd rather have a slightly hurried live calf than any dead one). It is SO hard to watch them work and not want to help, or to think that things aren't progressing fast enough. Locked in our out in the field, we often make a point to look only at 15 minute intervals, get an approx time of contractions & rest periods and seeing how "hard" the pushing is if we are watching the whole time and absolutely making a point to look at the clock not estimate time. More than once we have guessed that 15 minutes have passed and worried when we weren't seeing progress when it was only 5 minutes when we looked at our watches. A watched pot never boils, a watched cow never calves...
A couple years ago we put one of our cows in the corral to calve because she was looking over the fences for a better hiding place a little too much and it was just getting dark (6-7 pm). She held off calving until 6 the next morning, every time we went out to check or even turned on the light to try to see from the house, she was laying down but would get up and pace until things went totally quiet. We finally gave up and went to bed and she had it without a problem. We didn't even consider pulling because we knew that she hadn't relaxed enough to try to have it.
My last heifer calved last night, I got home at 5 and the water had broken and the toes were just BARELY starting to show as she pushed. She was up and down tons and of course where she thought she should have the calf was right up against the fence where he'd shoot under the fence and be darn near on the road. I chased her away from that spot and luckily she didn't go too far. She was up and down some more and pushing hard, rolling on her side and then sitting up to rest and the feet were out just past the ankles for what seemed like an eternity and boy did they look big. She rested for a few minutes and rolled over and with two big pushes shot the calf out all at once - it was just before 6 pm. Man I love it when they go like that!!
