My midwife had her first baby at home and after labouring completely naturally for almost a day to full dilation had no urge to push at all. She got an acupuncturist around to stimulate the pushing urge. She says she knew it would have come naturally eventually (often your body is just taking a break before that next stage), but she didn't want to wait any longer. I think we're so used to managing birth on a timed schedule we're not sure how it would work otherwise.