Characterization of Changes in Volume, Osmolarity and Electrolyte Composition of Porcine Fetal Fluids During Gestation1

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Abstract
Allantoic fluid volume of the porcine conceptus undergoes dynamic changes throughout gestation. Volume (X̄ ± SEM) increases from Day 20 (3.7 ± 0.3 ml) to an initial peak at Day 30 (189.0 ± 5.6 ml) and then decreases to Day 40 (69.5 ± 4.6 ml). It increases to a second midgestation peak on Day 58 (451.3 ± 53.5 ml) and then steadily decreases to Day 112 (23.8 ± 5.5 ml). The sodium [Na] and potassium [K] concentrations in allantoic fluid undergo equally dynamic changes. During periods of increasing allantoic fluid volume the [Na]/[K] ratio is greater than 1, while during periods of decreasing volume the [Na]/[K] ratio is less than 1. Calcium [Ca] , chloride [C1], and bicarbonate [HCO3] concentrations and the osmolarity of allantoic fluid also change throughout gestation. Changes in osmolarity indicate that changes in ion concentration cannot be explained on the basis of concentration and dilution effects brought about by changes in allantoic fluid volume. Changes in amniotic fluid volume and ion composition during pregnancy are markedly different from those of allantoic fluid and indicate that the two fluid pools may be of different origin or subject to different regulatory mechanisms. Data on volume and composition of fetal urine, stomach fluid and yolk sac fluid indicate that these fluids are not major sources of allantoic or amniotic fluid, especially early in gestation. We conclude that the chorioallantois of the porcine placenta separates two fluid pools, plasma and allantoic fluid, which differ markedly in ionic composition. The chorioallantois may be intimately involved in regulating the water and electrolyte milieu in the porcine conceptus.