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Physiology of amniotic fluid volume regulation

INTRODUCTION

Amniotic fluid (AF) is the liquid that surrounds the fetus after the first few weeks of gestation. During much of pregnancy, AF is derived almost entirely from the fetus and has a number of functions that are essential for normal growth and development [1]:

  • It helps to protect the fetus from trauma to the maternal abdomen
  • It cushions the umbilical cord from compression between the fetus and uterus
  • It has antibacterial properties that provide some protection from infection
  • It serves as a reservoir of fluid and nutrients for the fetus
  • It provides the necessary fluid, space, and growth factors to permit normal development of the fetal lungs and musculoskeletal and gastrointestinal systems.

Aberrations in amniotic fluid volume (AFV), both low (oligohydramnios) and high (polyhydramnios), are associated with a multitude of pregnancy-related problems. As an example, one study examined the pregnancy outcome of 147 women with a sonographic diagnosis of oligohydramnios (amniotic fluid index <50 mm) [2]. The pregnancies were complicated by an increased incidence of labor induction (42 percent), intrapartum fetal heart rate decelerations (48 percent), low Apgar scores (6 percent), admission to a neonatal intensive care unit (7 percent), meconium aspiration (1.4 percent), small for gestational age infants (24 percent), earlier delivery, and congenital malformations (10 percent).

Another report assessed the adverse effects of persistent polyhydramnios in 65 women [3]. The women had increased rates of both maternal problems (eg, diabetes, pregnancy-induced hypertension) and obstetrical complications (eg, preterm delivery, abnormal fetal presentation, fetal anomalies).

A solid understanding of the basic physiological mechanisms that regulate both AFV and composition are required to devise effective management strategies for pregnancies complicated by disorders of AFV. Current knowledge in these areas is limited, but progress is being made.
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