Amniotic fluid is the protective liquid that surrounds the developing fetus within the amniotic sac throughout pregnancy. This dynamic biological environment changes composition and volume as the pregnancy progresses. Monitoring the fluid volume is a routine part of prenatal care, particularly in later stages of gestation. Clinicians rely on the standardized Amniotic Fluid Index (AFI) to quantify this volume, providing a numerical score to assess fetal well-being. The AFI helps identify when the volume falls outside the expected range, signaling a need for closer monitoring or intervention.
The Essential Functions of Amniotic Fluid
The liquid environment performs several functions foundational for healthy fetal development. Amniotic fluid acts as a physical buffer, cushioning the fetus against external impacts or pressure. It also helps maintain a stable temperature, protecting the fetus from fluctuations in maternal body temperature.
The fluid is necessary for proper musculoskeletal growth, providing space for the fetus to move freely, which is integral for developing muscles and bones. By continuously swallowing and “breathing” the fluid, the fetus practices using muscles needed after birth, supporting the development of the fetal lungs and the gastrointestinal system. Furthermore, the fluid protects the umbilical cord from compression. The fluid also contains antibodies, offering protection against potential infections.
How the AFI is Measured
The AFI is a quantitative estimation of amniotic fluid volume obtained through a non-invasive ultrasound examination. To calculate this index, the clinician divides the pregnant uterus into four imaginary quadrants using the mother’s umbilicus as a central reference point. These quadrants are defined by vertical and horizontal lines passing through the navel.
In each section, the ultrasound probe locates the single deepest pocket of amniotic fluid. The measurement must be a vertical column of fluid unobstructed by fetal limbs or loops of the umbilical cord. Only the vertical depth of this pocket is measured in centimeters. The final AFI score is determined by adding the four individual measurements from each quadrant. This cumulative score is then compared against established reference ranges relative to the gestational age.
Defining Low and High Fluid Levels
A normal AFI range varies depending on the stage of pregnancy, though a typical range for the second and third trimesters is between 8 and 18 centimeters (cm).
Oligohydramnios (Low Fluid)
When the AFI falls below the normal threshold, the condition is referred to as oligohydramnios. This is commonly defined as an AFI less than 5 cm, or a measurement below the fifth percentile for gestational age. The reduced fluid volume increases the risk of umbilical cord compression, which limits the flow of oxygen and nutrients to the fetus. The lack of space can also lead to restricted fetal growth and, in earlier onset cases, affect lung development (pulmonary hypoplasia). Causes include premature rupture of membranes, placental issues, or problems with fetal kidney development.
Polyhydramnios (High Fluid)
Conversely, an excessive amount of amniotic fluid is known as polyhydramnios, typically diagnosed when the AFI is 24 cm or higher. This condition can stem from various factors, including maternal diabetes, multiple gestation pregnancies, or fetal issues that impair the ability to swallow the fluid. Risks associated with polyhydramnios include an increased chance of preterm labor due to uterine over-distention. The large volume of fluid also allows for greater fetal mobility, increasing the risk of the baby being in a breech or transverse position at labor. In rare cases, a sudden gush of fluid upon rupture of membranes can lead to an umbilical cord prolapse.
Clinical Management of Abnormal AFI
When an abnormal AFI is confirmed, management centers on increased surveillance to monitor fetal well-being. This involves frequent serial ultrasounds to track fluid volume changes, along with non-stress tests or biophysical profiles to assess fetal heart rate and movement.
Management of Oligohydramnios
For oligohydramnios, non-invasive interventions such as encouraging maternal hydration (orally or intravenously) may be attempted, as this can sometimes temporarily increase fluid volume. If low fluid is detected during labor, amnioinfusion may be performed. This involves introducing saline solution into the amniotic cavity to create a cushion, helping relieve pressure on the umbilical cord.
Management of Polyhydramnios
Management focuses on monitoring severity and addressing underlying causes, such as optimizing blood sugar control in diabetic mothers. If excess fluid causes severe maternal discomfort or significantly increases complication risk, amnioreduction may be considered. This procedure involves using a needle to drain a portion of the excess fluid to alleviate pressure.