I find it is best to always respect any cow that has just calved...whether it is a first calver or a tenth calver. As has been mentioned, there are many things that can affect how they will act. Most quiet cows will act quietly when they calve, but there are exceptions, and it is the exceptions are the reason to use some caution. Several years ago, a neighbor was killed by a cow that was a family pet... a cow that had been shown extensively. I do not want to create fear here, but just some old fashioned respect. Another guy who lives close to here, went out to check his cows in the morning, and found a newborn outside. The cow seemed normal so he bent over to see if he could get the calf to stand up so he could get it to the barn. As he leaned over, another cow bumped him from behind, and he went over on his head, and broke his neck. In the meantime, his wife had gone to work. He layed outside in -30 degrees for 10 hours before she returned home and realized he had not ate his breakfast.He had severe frostbite to his face, ears, hands and feet, but did live. Again, I do not want to impose fear with these stories, but respect. I honestly think animals can sense if you fear them. Treat them with respect and once the baby is born and OK, allow the mom and baby to have some alone time to bond.
I had a quiet cow calve quite uneventful, a few years ago. She got up and started to lick off the new born calf, and suddenly, she let the strangest beller out and started tossing the calf with her head and beating it with her feet. It was beyond anything I had ever seen before. I grabbed a fork and was able to get her to back off so that I could grab a leg of the calf and get it into safety. Fortunately, the calf had no permanent injuries, but was probably pretty bruised for awhile. A few seconds after I seperated them, she snapped back to normal again, and wanted her calf again. My vet told me that an occasional cow will go into a post partem shock situation, in which they can kill their calf. This was the only one I ever saw, and it was pretty scary. I kept this cow for several more years and she was excellent at calving time after that.