Maternal plasma hypertonicity is accentuated in the postterm rat

In humans and rats, pregnancy-associated maternal plasma volume expansion and plasma hypotonicity may facilitate maternal-to-fetal water transfer. Although reduced amniotic fluid volume occurs commonly in postterm pregnancy, the mechanisms are unknown. We previously demonstrated a reversal of pregnancy-induced maternal plasma hypotonicity that occurs in the near term (20 days) pregnant rats. We sought to determine whether the relative maternal plasma hypertonicity continues in the postterm period.

Rat gestation (normal, 21 days) was prolonged with subcutaneous progesterone injection. Pregnant rats at gestation, 18 days, 21 days, and 24 days and nonpregnant rats were studied. Maternal and fetal hematocrit levels, plasma osmolality, electrolyte levels, and amniotic fluid volume were determined. In addition, maternal and fetal tissues were analyzed for water and electrolyte content.

Compared with term (21days), postterm pregnant rats (24 days) had a significant increase in maternal and fetal plasma osmolality (293.7+/-1.4 mOsm/kg vs 302.8+/-3.7 mOsm/kg and 301.0+/-2.0 mOsm/kg vs 310.3+/-3.2 mOsm/kg, respectively; P<.01) and sodium and chloride concentrations. Conversely, both maternal and fetal hematocrit levels decreased significantly in the postterm period. Postterm rats demonstrated an increased fetal mortality rate (24%) and a significantly reduced amniotic fluid volume (4.2+/-0.6 mL vs 6.6+/-0.6 mL, P<.01).

These results indicate that the near-term reversal of maternal plasma hypotonicity that has been observed previously is further accentuated in the postterm pregnancy. This continued hypertonicity may induce a fetal-to-maternal water flow and contribute to postterm oligohydramnios and increased fetal morbidity and mortality rates.