Our aim was to determine the effect of maternal hyperthermia, both with and without maternal water deprivation, on fetal fluid balance. Seven pregnant ewes (131.8 +/- 1.0 days gestation) were studied during a control period and periods of maternal heating (MH, 42-44 degrees C for 8 h, water freely available), maternal water deprivation (MWD, 30 h) and maternal heating combined with water deprivation (MH + MWD, 30 h deprivation with heating during last 8 h). Relative to control values, MH increased maternal water intake and urine output, and [K+] in fetal plasma and fetal urine. Relative to control values, MH decreased maternal plasma osmolality, [Na+] and [K+]; fetal plasma osmolality and [Na+]; fetal lung liquid [Na+] and [Cl-]; and fetal production rates of lung liquid and urine. In response to MH + MWD, the osmolality, [Na+] and [Cl-] of maternal and fetal plasma, fetal lung liquid and fetal urine (excluding urinary [Cl-]) increased compared with control values. In the fetus, MH + MWD increased plasma and urinary [K+], and decreased production rates of lung liquid and urine compared with control values. During MH + MWD, compared with MH alone, greater alterations were seen in maternal rectal temperature, water input and urine output; osmolality, [Na+] and [Cl-] of maternal and fetal plasma, fetal lung liquid and fetal urine (excluding urinary [Cl-]); and fetal urinary [K+]. During MH + MWD, compared with MWD alone, greater alterations were seen in maternal plasma [Cl-] and [K+]; fetal urinary osmolality and [K+]; and fetal plasma [K+]. Our results show that, when water is available, maternal hyperthermia stimulates ewes to drink substantially more than under normal conditions, thereby decreasing their plasma osmolality; water transfer to the fetus may increase, thereby decreasing fetal plasma osmolality. When drinking water is unavailable, maternal hyperthermia and associated dehydration may decrease water transfer to the fetus. Thus, the fetus becomes not only hyperthermic, but also hyperosmotic and possibly hypovolaemic. Maternal hyperthermia, irrespective of the availability of drinking water, decreases production rates of lung liquid and urine in the fetus.

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