Relationship between Epo and Hct at HA vs SL in the nonpregnant state and during pregnancy. Linear regression models were used to test whether the relationship (slope) between Epo and Hct differed between Andean women residing at HA vs SL in the nonpregnant, postpartum period or during pregnancy (20 or 36 weeks). Complete Epo and Hct data were available for HA (n = 28) and SL (n = 16) women at all study points. Regression lines and individual data points for HA (solid lines, solid squares) and SL (dashed lines, open circles) are shown. (A) At SL, Epo and Hct levels were unrelated in the nonpregnant state; in contrast, nonpregnant Epo and Hct values were inversely associated at HA. (B) At 20 weeks, the relationship between Epo and Hct values was significantly different between the two altitudes, as indicated by the HA-SL slope contrast (P = 0.04). Specifically, at 20 weeks, Epo and Hct values were inversely related at HA and were positively associated at SL. (C) At 36 weeks, the relationship between Epo and Hct was similar to that of the nonpregnant state, with an inverse association apparent at HA and no association at SL.