The value of urine osmolality as an index of stress in the ovine fetus

In ovine fetuses, during 100-130 days of gestation, urine osmolalities less than 175 mosmol/kg water were associated with plasma immunoreactive adrenocorticotrophin (ACTH) concentrations below 40 pg/ml in 40/41 samples. In 18/29 fetuses with urine osmolalities greater than 175 mosmol/kg water plasma ACTH was significantly elevated. In 38 samples of fetal blood there was a significant correlation between plasma ADH and ACTH concentrations. By least squares regression the equation to the line was [ACTH] = 5.06 + 3.70 [ADH] (r = 0.62, P<0.001). In samples from fetuses of gestational ages 100-140 days, with urine osmolalities of 302 ± 86 mosmol/kg (mean ± SD) the blood pH, pO₂ and pCO₂ values were not significantly different from those in 50 samples from fetuses with urine osmolalities of 125 ± 22 mosmol/kg. It is proposed that the measurement of fetal urine osmolality provides a good index of fetal stress. A fetus with a urine osmolality<175 mosmol/kg is almost invariably in the optimum, unstressed condition.