The amniotic fluid index is a standardized way to assess the sufficiency of amniotic fluid quantity in pregnancy. The amniotic fluid index is used in patients who are at least 24 weeks pregnant with a singleton gestation.
Decreased fetal urine output can have a number of causes, which fall into two 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 shunting of fetal blood flow away from the kidney to the heart and brain. It is the same mechanism which causes oliguria in critically ill adults.
When the fetus receives adequate nutrients and oxygen from the placenta, blood is shunted away from the fetal kidney, glomerular filtration rate decreases, and urinary output decreases. Therefore, decreased amniotic fluid volume due to decreased urine production by the fetal kidney is a reflection of chronic hypoperfusion of the fetus.
Oligohydramnios can also occur because the patient's amniotic membrane has ruptured and amniotic fluid is leaking out of the uterus.
The normal fetus is constantly swallowing amniotic fluid and urinating to create more fluid. If the fetus is unable to swallow the typical amounts of amniotic fluid, this can lead to polyhydramnios. This can occur due to gastrointestinal malformations, fetal neurologic problems such as anencephaly, or mechanical obstruction of the esophagus by other intrathoracic processes.
Increased amniotic fluid production occurs as a result of fetal polyuria, such as in uncontrolled maternal diabetes with persistently elevated maternal blood sugars. In these cases, it may be associated with fetal macrosomia.
Many cases of polyhydramnios are idiopathic, meaning no definite cause is identified.
Since amniotic fluid is primarily made up of fetal urine, low amniotic fluid volume, or oligohydramnios, typically indicates either fetal urine output or leakage of amniotic fluid from the uterus, such as when the patient's water breaks.
Polyhydramnios, or increased amniotic fluid volume, also has a number of potential causes, with two primary common mechanisms: decreased fetal swallowing of amniotic fluid, or increased fetal production of amniotic fluid. Polyhydramnios can lead to overdistension of the gravid uterus, especially in cases where the fetus is normal size or large for dates, which increases the patient's risk for preterm contractions and preterm delivery, as well as 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.
The uterus should be divided into four quadrants to assess 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 ultrasound, and should be maintained in an axial plane (notch to the patient's right). This ensures that each pocket of fluid is being measured in the same plane. The deepest vertical pocket of fluid in each quadrant should be identified and measured, and these four 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 are not always well seen in B-mode (standard 2D greyscale) imaging. The calipers may not cross over any segments of the umbilical cord or any fetal parts.
Alternative Measurements of Amniotic Fluid Volume
In pregnancies less than 24 weeks, or with multiple gestations, a single deepest pocket is used. The technique used to measure a single deepest pocket (also referred to as a maximum vertical pocket) is identical to the measurement of amniotic fluid amounts in the four 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 cm to 8 cm (less than 2 cm is oligohydramnios, greater than 8 cm is polyhydramnios).
Recall that, in multiple gestations, a single deepest pocket is measured rather than an amniotic fluid index. In twins which share a placenta but which have separate amniotic sacs (monochorionic, diamniotic twins), a sub-type of identical twins, a condition called twin to twin transfusion syndrome (TTTS) may be seen. In this condition, placental blood flow is shunted disproportionately to one twin. The recipient twin becomes overloaded with fluid and attempts to compensate by increasing urine production, leading to increased amniotic fluid volume or polyhydramnios. The donor twin receives inadequate perfusion and shunts blood flow away from the kidneys, leading to decreased amniotic fluid volume or oligohydramnios. This so-called "poly-oly syndrome" is the earliest stage of twin to twin transfusion syndrome. The condition can progress from this point, eventually leading to severe fluid overload and heart failure of the recipient twin, and growth restriction of the donor twin. The death of one or both twins may occur.
Amniotic fluid volume is also part of the fetal biophysical profile, a special type of ultrasound used to assess fetal well-being. The biophysical profile has four sonographic components, each of which must be seen within 30 minutes of starting the ultrasound:
Fetal breathing (continuous movement of the fetal diaphragm for at least 30 seconds)
Fetal movement (at least three discrete movements of the fetal body or limbs)
Fetal tone (at least one active extension of a fetal limb with return to flexion, or opening and closing of the fetal hand)
Amniotic fluid volume (single deepest pocket of at least 2 cm)
If available, a fetal non-stress test (fetal heart rate tracing) is also performed with the biophysical profile, for a total of five components.
A healthy, term fetus that is not under physiologic stress would be expected to demonstrate all four of these behaviors on ultrasound. Fetuses which are preterm may not display all of these behaviors, so management of pregnancies in which the fetus does not demonstrate all four of the behaviors described depends on the gestational age, and on which factors are abnormal.
A normal amniotic fluid index is 5 cm to 25 cm. Less than 5 cm is considered oligohydramnios, and greater than 2cm is considered polyhydramnios.