Technically, it'd be both. I also fail to see why it's really all that "absurd" to suggest that stress could be a negative factor here.
The simple fact of the matter is that stress produces adrenaline, and adrenaline is exactly the opposite of what women need during childbirth. It causes muscles to tense up, rather than relax, and it increases blood pressure.
When you're trying to push a foreign object the size of a football through a hole the size of a lemon (without ripping yourself wide open or giving yourself an aneurysm in the process), and expecting to take hours doing so, none of those effects are good things to have.
Are there tons of spazzy strangers around a woman during a hospital delivery anyway, and does this cause stress?
Absolutely.
However, that's exactly why a lot of experts have started to suggest that hospitals (as they are set up now, anyway) really might not be a great environment for labor and delivery in the first place.
For most of human history, women would have given birth at home, with only one or two close family members and maybe a mid-wife near-by. That would have been massively less stressful and frightening for everyone involved.
