Background Whether the fetus can experience pain remains controversial. During the last half of pregnancy, the neuroanatomic connections for nociception are in place, and the human fetus mounts sizable stress responses to physical insults. Analgesia has been recommended for intrauterine procedures or late termination, but without evidence that it works. The authors investigated whether fentanyl ablates the fetal stress response to needling using the model of delayed interval sampling during intrahepatic vein blood sampling and transfusion in alloimmunized fetuses undergoing intravascular transfusion between 20 and 35 weeks. Methods Intravenous fentanyl (10 microg/kg estimated fetal weight x 1.25 placental correction) was given once at intrahepatic vein transfusion in 16 fetuses, and changes (posttransfusion - pretransfusion) in beta endorphin, cortisol, and middle cerebral artery pulsatility index were compared with intrahepatic vein transfusions without fentanyl and with control transfusions at the placental cord insertion. Results Fentanyl reduced the beta endorphin (mean difference in changes, -70.3 pg/ml; 95% confidence interval, -121 to -19.2; P = 0.02) and middle cerebral artery pulsatility index response (mean difference, 0.65; 95% confidence interval, 0.26-1.04; P = 0.03), but not the cortisol response (mean difference, -10.9 ng/ml, 95% confidence interval, -24.7 to 2.9; P = 0.11) in fetuses who had paired intrahepatic vein transfusions with and without fentanyl. Comparison with control fetuses transfused without fentanyl indicated that the beta endorphin and cerebral Doppler response to intrahepatic vein transfusion with fentanyl approached that of nonstressful placental cord transfusions. Conclusions The authors conclude that intravenous fentanyl attenuates the fetal stress response to intrahepatic vein needling.