Amniotic Fluid Index(Archived)

Megan Lord; Sarah Marino; Martha Kole

Amniotic Fluid Index(Archived)

Archived, for historical reference only

Introduction

Throughout intrauterine development, the fetus exists in a compartment filled with amniotic fluid, allowing free movement.[1] This fluid acts as a protective cushion against potential injuries, helps maintain a stable temperature, and is inhaled and swallowed by the fetus, contributing to normal lung development.[2] Amniotic fluid is a dynamic substance, with its volume and chemical makeup regulated within narrow limits.[3] The amount of amniotic fluid gradually increases until approximately 34 weeks of gestation, after which it slightly decreases until the 40th week and then declines more sharply until the 42nd week.[4]

The volume at any given moment is a function of several interrelated fluid fluxes. In the early stages of pregnancy, the composition of amniotic fluid resembles a complex dialysate derived from maternal serum. As the fetus grows, changes occur in the fluid's composition; notably, sodium concentration and osmolality decrease while urea, creatinine, and uric acid levels increase.[5][6]

Amniotic fluid also contains various steroid and protein hormones.[7][8] Initially, it has little to no particulate matter, but by 16 weeks of gestation, a significant number of cells shed from the amnion, skin, and tracheobronchial tree are present. These cells are crucial for antenatal diagnosis and serve as a source of DNA for karyotype analysis following amniocentesis.[9]

Typically, fetuses do not defecate during pregnancy; however, if under severe stress, they may pass meconium. This material contains bile pigments that can stain the amniotic fluid green, indicating fetal stress.[10]

The amniotic fluid index is a standardized method used to measure the amount of amniotic fluid during pregnancy.[11][12] This measurement helps evaluate fetal well-being and detect potential complications such as oligohydramnios or polyhydramnios. The amniotic fluid index is typically performed on patients who are at least 24 weeks pregnant with a singleton gestation.[13][14]

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.[15] 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.[16][17]

Oligohydramnios

When the fetus receives insufficient nutrients and oxygen from the placenta, blood is redirected away from the fetal kidney, decreasing the glomerular filtration rate and reducing 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.[18][19]

Another common cause of oligohydramnios is the rupture of the amniotic membrane, which allows amniotic fluid to leak out of the uterus.[20]

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.[21]

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.[22]

Pathophysiology

Oligohydramnios

The amniotic fluid is primarily made up of fetal urine. 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.[23]

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.[21] 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.[24][25]

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.[26] 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 visible in B-mode (standard two-dimensional greyscale) imaging. The calipers may not cross over any segments of the umbilical cord or fetal parts.[19][27]

Alternative Measurements of Amniotic Fluid Volume

A single deepest pocket is used in pregnancies less than 24 weeks or with multiple gestations.[28] 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.[29][30]

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:[31][32]

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.[33]

A healthy, term fetus that is not under physiological stress is expected to demonstrate all 4 of these behaviors on ultrasound. Preterm fetuses 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.[34]

Clinical Significance

The normal range of amniotic fluid volume changes with gestational age. The median amniotic fluid index (AFI) is approximately 14 cm between 20 and 35 weeks of gestation. An AFI below 5 cm indicates oligohydramnios, while the 5th percentile cutoff for the second and third trimesters is about 7 cm. Polyhydramnios is generally diagnosed when the AFI exceeds 24 or 25 cm, depending on local guidelines.[1]

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.

Details

References

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