Amniotic Fluid Index

Etiology and Epidemiology

Decreased fetal urine output can arise from several causes, which fall into 2 general categories—fetal urinary tract obstruction and decreased urine production by the fetal kidney. Urinary tract obstruction can occur anywhere along the fetal urinary tract and can be catastrophic for the fetus. Decreased urine production by the fetal kidney typically reflects inadequate blood flow to the fetal kidney, caused by the shunting of fetal blood flow away from the kidney to the heart and brain. This mechanism mirrors the cause of oliguria in critically ill adults.[5][6][7]

Oligohydramnios

When the fetus receives insufficient nutrients and oxygen from the placenta, blood is redirected away from the fetal kidney, leading to a decrease in the glomerular filtration rate and a reduction in urinary output. Therefore, decreased amniotic fluid volume due to decreased urine production by the fetal kidney is a reflection of chronic hypoperfusion of the fetus.

Another common cause of oligohydramnios is the rupture of the amniotic membrane, which allows amniotic fluid to leak out of the uterus.

Polyhydramnios

The normal fetus constantly swallows amniotic fluid and urinates to maintain the fluid level. If the fetus is unable to swallow the typical amounts of amniotic fluid, it can lead to polyhydramnios. This condition can occur due to gastrointestinal malformations; fetal neurological issues, such as anencephaly; or mechanical obstruction of the esophagus by other intrathoracic processes.

Increased production of amniotic fluid can occur due to fetal polyuria, often observed in cases of uncontrolled maternal diabetes with persistently elevated maternal blood sugar levels. In these cases, it may be associated with fetal macrosomia. Many cases of polyhydramnios are idiopathic, meaning no definite cause is identified.

Pathophysiology

Oligohydramnios

The amniotic fluid is primarily made up of fetal urine, so low amniotic fluid volume, known as oligohydramnios, typically indicates either decreased fetal urine output or leakage of amniotic fluid from the uterus, which often occurs when the patient's water breaks.

Polyhydramnios

Polyhydramnios, or increased amniotic fluid volume, also has several potential causes, with 2 primary mechanisms—decreased fetal swallowing of amniotic fluid or increased fetal production of amniotic fluid. Polyhydramnios can cause overdistension of the gravid uterus, especially in cases where the fetus is normal size or large for dates. This overdistension increases the patient's risk for preterm contractions, preterm delivery, and premature rupture of membranes, in which the patient's water breaks before the onset of labor. Overdistension of the uterus is also a risk factor for postpartum hemorrhage after delivery.

Specimen Requirements and Procedure

The uterus should be divided into 4 quadrants to assess the amniotic fluid index. Each quadrant should be examined systematically. The ultrasound transducer should be held perpendicular to the patient's spine, not perpendicular to the patient's skin, as is performed in most other ultrasounds, and should be maintained in an axial plane (notch to the patient's right). This technique ensures that each fluid pocket is measured in the same plane. The deepest vertical pocket of fluid in each quadrant should be identified and measured, and these 4 measurements should be added together to calculate the total amniotic fluid index. Calipers should be oriented vertically. Color Doppler is typically placed over the pocket of fluid to ensure that the pocket does not contain any segments of the umbilical cord, which may not be clearly visible in B-mode (standard two-dimensional greyscale) imaging. The calipers may not cross over any segments of the umbilical cord or fetal parts.

Alternative Measurements of Amniotic Fluid Volume

A single deepest pocket is used in pregnancies less than 24 weeks or with multiple gestations. The technique used to measure a single deepest pocket, also called a maximum vertical pocket, is identical to the measurement of amniotic fluid amounts in the 4 quadrants used to determine an amniotic fluid index. The entire uterus should be examined, and the single deepest vertical pocket of fluid should be identified and measured. A normal single deepest pocket is 2 to 8 cm. A measurement less than 2 cm indicates oligohydramnios, whereas a measurement greater than 8 cm is polyhydramnios.[8][9]

Results, Reporting, and Critical Findings

Biophysical Profile

Amniotic fluid volume is an important component of the fetal biophysical profile, a specialized ultrasound technique used to assess fetal well-being. The biophysical profile has 4 sonographic components, each of which must be observed within 30 minutes of starting the ultrasound:

• Fetal breathing: Continuous movement of the fetal diaphragm for at least 30 seconds)

• Fetal movement: At least 3 discrete movements of the fetal body or limbs)

• Fetal tone: At least 1 active extension of a fetal limb with the return to flexion or opening and closing of the fetal hand

• Amniotic fluid volume: A single deepest pocket of at least 2 cm

If available, a fetal non-stress test, which measures fetal heart rate tracing, is also performed with the biophysical profile for 5 components.

A healthy, term fetus that is not under physiological stress is expected to demonstrate all 4 of these behaviors on ultrasound. Fetuses that are preterm may not display all of these behaviors. Therefore, the management of pregnancies in which the fetus does not show all 4 behaviors described depends on the gestational age and the specific factors that are abnormal.

Clinical Significance

A normal amniotic fluid index is 5 cm to 25 cm using the standard assessment method. Less than 5 cm is considered oligohydramnios, and greater than 25  cm is considered polyhydramnios.

Enhancing Healthcare Team Outcomes

Healthcare professionals, including nurse practitioners and other healthcare providers caring for pregnant patients, must have a solid understanding of the amniotic fluid index. This index is an important indicator of fetal well-being and part of the biophysical profile. When there is suspicion of a problem during pregnancy, the patient should be referred to an obstetrician who may order an ultrasound to determine the amniotic fluid index. Persistently low levels of amniotic fluid index during pregnancy may be associated with a birth defect in the fetus.