However I do recall a conversation between some midwife friends where they talked about the smell changing in a labour room at the onset of 2nd stage, and using this as a diagnostic tool. I'll try to find a reference for this, but it would certainly suggest a link between scent and labour.
LOL... I know that smell. I would say it's like a combination of sex and wet grass.

I could just have a very sensitive nose, because I could always tell when my daughter was going to have a fit because I could smell a metalic scent on her breath and could tell my nephew was diabetic before he was diagnosed. I can't wear perfumes or shop-bought deodorants because they make me sick - that's why I know it's my scent and not something I've put on myself.