Development of temperature regulation in the fetal sheep.

To investigate the development of fetal thermoregulatory mechanisms the temperature of the ovine fetus in utero was altered by circulating water through a coil placed around the fetus. In 18 fetuses (76-142 days), temperature recordings were made from thermister probes placed in the fetal cranium below the frontal lobes, the fetal rectum, the amniotic cavity and the maternal vena cava. Severe hypothermia was observed during fetal surgery; the maximum decrement in fetal brain temperature ranged between -7.38 degrees C and -0.83 degrees C and was correlated inversely with gestational age (P less than 0.005). The temperature of the live fetal sheep in utero was 39.05 to 40.37 degrees C and was always greater than the maternal temperature by 0.30 to 0.78 degrees C. The amniotic fluid was intermediate in temperature between the fetus and mother. During cooling studies the fetal cranial temperature fell more quickly and to a greater extent (P less than 0.05) than the fetal rectal temperature. When the cooling was continued for 60 min, a steady fall in fetal temperature was observed until a plateau was reached at 20 to 40 min. The recovery time increased with increasing gestational age (P less than 0.001) suggesting that thermoregulation is still immature in late gestation. During prolonged cooling, fetal hypoxaemia and acidosis were observed.