The Amniotic Fluid Index (AFI) is a common measurement used during pregnancy to assess the volume of fluid cushioning the developing fetus. This fluid provides protection, supports growth, and facilitates the development of various fetal systems. Monitoring the amount of this fluid is a standard procedure in prenatal care, as deviations from the expected range can signal potential issues with fetal health or placental function. The AFI measurement is a non-invasive tool used to assess fetal well-being throughout the second and third trimesters.
What is Amniotic Fluid and How is AFI Measured
Amniotic fluid is a clear, water-like substance primarily composed of fetal urine in the latter half of pregnancy. It protects the fetus from external trauma and helps maintain a stable temperature inside the womb. The fluid provides space for the fetus to move, aiding in the development of muscles and skeletal structure. The fetus also breathes and swallows the fluid, a recycling mechanism crucial for the maturation of the lungs and gastrointestinal tract.
The Amniotic Fluid Index is calculated using an ultrasound technique that quantifies the fluid volume. The procedure divides the uterus into four imaginary quadrants, measuring the deepest, vertical pocket of fluid in each section. These measurements are taken in centimeters, ensuring no fetal parts or umbilical cord loops are within the pocket. The final AFI score is the sum of these four deepest vertical measurements, providing a single composite number that estimates the total fluid volume.
Defining Normal AFI Ranges
Generally, an AFI between 5 centimeters and 25 centimeters is considered within the broad normal limits for a singleton pregnancy in the second and third trimesters. A more specific normal range often cited falls between 8 centimeters and 18 centimeters, reflecting the median fluid volume across most of the pregnancy. A measurement below the lower threshold indicates low fluid, a condition called Oligohydramnios. A score above the upper limit suggests excess fluid, known as Polyhydramnios. The median AFI level naturally peaks around 14 centimeters between 20 and 35 weeks, beginning to decrease as the pregnancy approaches full term.
Low Amniotic Fluid Readings (Oligohydramnios)
Oligohydramnios is diagnosed when the AFI score falls below 5 centimeters. This low fluid volume can be caused by several factors, including a leak or rupture of the amniotic membranes, often called “water breaking.” Placental problems that reduce blood flow and nutrients can also trigger Oligohydramnios, causing the fetus to reduce urine production.
Fetal causes often involve issues with the development of the kidneys or urinary tract, such as renal agenesis, limiting the fetus’s ability to produce urine. The condition is also more common in post-term pregnancies extending beyond 42 weeks due to declining placental function. Low AFI increases the risk of complications, particularly umbilical cord compression, where the lack of fluid cushioning restricts blood flow and oxygen. Severe Oligohydramnios diagnosed early in the second trimester can impair lung development, resulting in pulmonary hypoplasia.
High Amniotic Fluid Readings (Polyhydramnios)
Polyhydramnios is diagnosed when the AFI exceeds 24 or 25 centimeters, indicating an excessive volume of amniotic fluid. The most common cause is often classified as idiopathic, meaning no specific cause can be identified, especially in mild cases. When a cause is found, it frequently relates to the fetus’s inability to swallow or process the fluid it produces. For instance, congenital anomalies causing a blockage in the gastrointestinal tract, such as esophageal or duodenal atresia, can prevent the fluid from being recycled properly.
Maternal health conditions are also a significant contributor, with poorly controlled maternal diabetes being a primary risk factor, as high blood sugar levels can lead to increased fetal urination. Other causes include multiple gestation, where conditions like twin-to-twin transfusion syndrome can result in one twin having Polyhydramnios. The excess fluid puts physical pressure on the uterus and surrounding organs, which can lead to maternal shortness of breath, preterm labor, premature rupture of membranes, and an increased risk of placental abruption.
Monitoring and Intervention for Abnormal AFI Scores
When an abnormal AFI score is recorded, the clinical response is heightened surveillance of the pregnancy. For Oligohydramnios, this involves more frequent monitoring of fetal well-being through non-stress tests (NSTs) and biophysical profiles (BPPs), which assess the fetus’s heart rate, movement, muscle tone, and breathing. In severe cases, a procedure called amnioinfusion may be considered, where a saline solution is introduced into the amniotic sac to temporarily increase fluid volume. This intervention is used to improve visualization during monitoring or to cushion the umbilical cord during labor.
For Polyhydramnios, management depends on the severity of the fluid excess and the underlying cause. Mild cases require regular monitoring to ensure the condition does not worsen and that fetal growth remains appropriate. More severe cases may involve draining excess fluid, a procedure known as amnioreduction, to reduce the risk of preterm labor and maternal discomfort. Healthcare providers must balance the risks of continuing the pregnancy with the risks associated with an early delivery, guided by ongoing fetal surveillance tests.