beta-endorphin was measured by radioimmunoassay in peripheral plasma of nonpregnant women ( 58 +/- 2.4 pg/ml , n = 17 , mean +/- se ) , during the first trimester ( 47 +/- 2.4 pg/ml , n = 11 ) , the second trimester ( 33 +/- 1.9 , n = 11 ) , and the third trimester ( 49 +/- 2.7 pg/ml , n = 10 ) of pregnancy , during early ( 202 +/- 32 pg/ml , n = 12 ) and advanced labor ( 389 +/- 78 pg/ml , n = 10 ) , and 30 to 60 minutes post partum ( 177 +/- 22 pg/ml , n = 12 ) .
C0032981
third trimester
C0032961
pregnancy