AFI stands for amniotic fluid index, a measurement taken during a pregnancy ultrasound to estimate how much fluid surrounds the baby. It’s reported in centimeters, and a normal value generally falls between 8 and 24 cm. Your provider uses this number to check whether you have too little or too much amniotic fluid, both of which can signal issues that need closer monitoring.
How AFI Is Measured
To calculate the AFI, the sonographer mentally divides your uterus into four quadrants using your belly button and a vertical midline as reference points. In each quadrant, they measure the deepest vertical pocket of fluid that doesn’t contain the baby’s limbs or umbilical cord. Those four measurements are added together, and the total is your AFI.
During the measurement you’ll lie on your back while the sonographer holds the ultrasound probe perpendicular to the floor. The process takes just a few minutes and is part of a standard ultrasound exam. Some women feel mild discomfort from the probe pressure or lightheadedness from lying flat, since the weight of the uterus can compress a large vein. If that happens, a rolled towel placed under one hip to tilt you slightly to the left usually helps.
Normal AFI Ranges by Week
Amniotic fluid volume isn’t static. It rises steadily from around 13 weeks, peaks near 34 to 36 weeks, and then gradually declines toward your due date. In the third trimester, the average AFI is about 16 cm. Here’s what typical ranges look like in late pregnancy:
- 34 weeks: roughly 11.7 to 17.3 cm (5th to 95th percentile)
- 36 weeks: roughly 10.6 to 15.7 cm
- 38 weeks: roughly 9.8 to 14.7 cm
- 40 weeks: roughly 8.7 to 13.7 cm
The gradual drop after 38 weeks is normal. Your provider compares your number to the expected range for your specific gestational age rather than using a single cutoff for all stages of pregnancy.
What a Low AFI Means
An AFI of 5 cm or less is classified as oligohydramnios, the medical term for low amniotic fluid. Values between 5 and 8 cm are considered “low normal” and may prompt extra monitoring even if they don’t meet the formal threshold.
Several things can cause low fluid levels. The most common include a rupture in the membranes (your water partially breaking, sometimes as a slow leak rather than a dramatic gush), problems with blood flow through the placenta, chronic high blood pressure, diabetes, and certain fetal conditions that affect kidney function or growth. Some blood pressure medications can also reduce fluid production.
When the AFI is low, your provider will typically increase the frequency of ultrasound monitoring and may add Doppler studies, which measure blood flow through the umbilical cord and placenta. Depending on how far along you are and what’s causing the low fluid, the plan might range from closer surveillance and increased hydration to earlier delivery if the baby’s well-being is at risk.
What a High AFI Means
An AFI of 24 cm or higher is classified as polyhydramnios, meaning too much fluid. It’s further graded by severity:
- Mild: 24 to 29.9 cm
- Moderate: 30 to 34.9 cm
- Severe: 35 cm or above
Most cases are mild, and in many of those, no clear cause is found. When a cause is identified, gestational diabetes is one of the most common. Poorly controlled blood sugar can lead to the baby producing more urine, which increases fluid volume. Fetal anomalies that interfere with the baby’s ability to swallow amniotic fluid, such as blockages in the esophagus or upper intestine, can also be responsible. In identical twins sharing a placenta, a condition called twin-to-twin transfusion syndrome can cause one sac to have excess fluid while the other has too little.
Mild polyhydramnios often requires only periodic rechecking. More significant cases may involve additional testing to look for an underlying cause, including a glucose tolerance test if gestational diabetes hasn’t already been ruled out, and a detailed anatomy scan of the baby.
AFI vs. Single Deepest Pocket
You might see a different measurement on your ultrasound report: the single deepest vertical pocket, sometimes abbreviated SDP or DVP. Instead of measuring four quadrants and adding them up, this method measures just the one deepest pocket of fluid. A normal single deepest pocket is 2 to 8 cm.
Both methods are widely used, and neither is clearly better at predicting problems for the baby. However, a large Cochrane review found that using the four-quadrant AFI flags more cases of low fluid and leads to significantly more labor inductions and cesarean deliveries for fetal distress, without actually improving outcomes for newborns. Because of this, many practices now prefer the single deepest pocket method for routine monitoring, since it results in fewer unnecessary interventions. If your report includes one measurement but not the other, that’s likely a reflection of your provider’s preferred approach.
Why Regular Monitoring Matters
A single AFI reading is a snapshot. Fluid levels can change over days or weeks, especially after 37 weeks when amniotic fluid naturally starts to decline more noticeably. For pregnancies that are otherwise uncomplicated, AFI is usually checked as part of routine third-trimester ultrasounds. For higher-risk pregnancies, your provider may measure it weekly or even twice weekly.
Research on term pregnancies with abnormal AFI has consistently found that early detection paired with Doppler blood flow studies leads to better outcomes for both mother and baby. Catching a trend toward low or high fluid early gives your care team time to investigate the cause, adjust the monitoring schedule, and plan the timing and method of delivery if needed. If your AFI comes back outside the normal range, it doesn’t automatically mean something is wrong with the baby. It means your provider has one more piece of information to guide what happens next.