Figure 1. Linear regression analysis. A reverse correlation was found between progesterone concentration and time after delivery (A) (r = -0.527; P = 0.036). However, no correlation was found between beta-endorphin concentration and time (B) (r = 0.089; P = 0.744). There was no correlation between isoflurane minimum alveolar concentration (MAC) and progesterone concentration (C) (r = 0.005; P = 0.985) or between isoflurane MAC and beta-endorphin concentration (D) (r = 0.154; P = 0.568).