To investigate if labor pain is affected by mother's hormone levels.

The hormonal effects of pregnancy are well documented. Although differences in pain sensitivity between women and men are partly attributable to social conditioning and to psychosocial factors, many laboratory studies of humans have described sex differences in sensitivity to noxious stimuli, suggesting that biological mechanisms underlie such differences. Some animal studies have suggested that sensitivity to pain decreases during pregnancy because of the progressive activation of endogenous pain inhibitory systems. This effect may be mediated by pregnancy-associated hormones, in particular progesterone.