Objective: Maternal plasma hypotonicity occurs early in rat and human pregnancy with resetting of the plasma osmolality threshold for vasopressin secretion and thirst. In humans, amniotic fluid volume reaches maximum levels in the mid-third trimester and decreases thereafter. We hypothesized that a reversal of maternal plasma hypotonicity occurs near term, contributing to reduced fetal and amniotic fluid water content.
Methods: Maternal plasma and amniotic fluid osmolality and sodium levels, including amniotic fluid volume, were measured at 16, 18 and 20 days of rat gestation. Additionally, maternal and fetal brains were analyzed for water and electrolyte content. Non-pregnant adult female rats represented controls.
Results: Compared to non-pregnant adults, 16-day and 18-day pregnant rats had significantly lower plasma osmolality (301.0 +/- 2.3 vs. 295.4 +/- 2.8 and 289.7 +/- 3.3 mOsm/kg, respectively) and sodium levels (140.3 +/- 1.0 vs. 135.7 +/- 0.8 and 133.4 +/- 1.4 mEq/l, respectively). Conversely, 20-day pregnant rats showed no significant difference in plasma osmolality (298.4 +/- 3.1 mOsm/kg) or sodium levels (137.6 +/- 1.0 mEq/l) from non-pregnant adults. With advancing gestation, the amniotic fluid volume decreased whereas the osmolality and sodium levels increased significantly. Maternal brain water content was significantly higher in 16-day and 18-day pregnant rats compared to control rats (78.7 +/- 0.1 and 78.1 +/- 0.2 vs. 76.9 +/- 0.2% wet weight) and returned to non-pregnant values in the 20-day pregnant rats (76.6 +/- 0.2%). In association with the maternal changes, fetal brain water and electrolyte content significantly decreased from 16-day to 18-day to 20-day fetuses.
Conclusion: These findings indicate a reversal of maternal plasma hypotonicity and reduced maternal brain water content in the near-term pregnant rat. We speculate that relative maternal plasma hypertonicity near term may contribute to reduced amniotic fluid volume.