Correlation of maternal plasma volume and composition with amniotic fluid index in normal human pregnancy

Early in human pregnancy, maternal plasma osmolality decreases and plasma volume begins to increase. Continued plasma volume expansion is linked to fetal fluid acquisition, and ultimately normal amniotic fluid volume. Although the mechanisms for these physiological responses are uncertain, we hypothesized that the time course of maternal plasma hypotonicity, plasma volume expansion and amniotic fluid index (AFI) were interrelated. We sought to examine these interrelationships during human pregnancy.

Normal primiparous pregnant patients had monthly determinations of plasma volume using Evans blue from the second trimester to term, and 8 weeks after delivery. At each visit, fasting, morning blood samples were also drawn to measure hematocrit, osmolality, electrolytes, urea, glucose and creatinine. AFI was determined by ultrasound.

Plasma volume, blood volume and AFI demonstrated significant increases with advancing gestational age. There were no significant changes in plasma sodium or osmolality from the second trimester to term. AFI was not associated with either plasma volume or osmolality.

Although similar factors may therefore be responsible for the initiation of plasma volume expansion and plasma hypotonicity, the magnitude and degree of change of these processes are probably under independent regulation. Individual patient variation in AFI is a result of factors other than maternal blood volume or composition.