Amniotic Fluid Index Chart: What the Numbers Mean at Every Week

The amniotic fluid index chart is a week-by-week map of normal amniotic fluid volume, and the standard reference range runs from about 5 to 25 cm across the entire second half of pregnancy, holding roughly steady from 20 to 40 weeks even as the actual fluid volume inside the sac rises and then falls. A measurement below 5 cm defines oligohydramnios; above 25 cm defines polyhydramnios. That single fixed range is what makes the chart easy to read and also why obstetricians stopped leaning on the amniotic fluid index as heavily as they once did.

What the Amniotic Fluid Index Actually Measures

The amniotic fluid index is a four-quadrant ultrasound sum. The sonographer divides the uterus into four roughly equal compartments using the linea nigra as the vertical divider and the umbilicus as the horizontal landmark. In each quadrant, they find the deepest vertical pocket of fluid free of fetal parts or umbilical cord. They measure that pocket in centimeters. Add the four numbers, and you have the amniotic fluid index.

A typical normal calculation looks like 4 cm plus 5 cm plus 3 cm plus 6 cm, giving an index of 18 cm. The four-quadrant method was introduced by Phelan and colleagues in 1987 as a standardized alternative to single-pocket measurement, and it became the dominant method in American obstetric practice through the 1990s and 2000s before newer evidence shifted practice.

The measurement is not the fluid volume. It is a two-dimensional proxy for a three-dimensional quantity. This distinction matters because a fetus wedged low in the pelvis, a fibroid displacing the uterus, or a very large baby can compress pockets and lower the number without any true fluid deficit. In plain terms: the index is a photograph of a shadow, not the object casting it.

The deepest vertical pocket is the simpler alternative. For that measurement, the sonographer finds the single largest pocket free of cord and fetal parts and measures only its deepest point. Normal is generally 2 cm or greater, with values of 2 to 8 cm treated as the usual comfortable range. The key difference between the two methods is how they fail: the four-quadrant index can be artificially low when one pocket is compressed, while the single deepest pocket is less sensitive to oligohydramnios but more prone to flagging polyhydramnios.

The Week-by-Week Amniotic Fluid Index Chart

The reference range is remarkably flat. The table below gives the commonly cited normal bands by gestational week along with the median, drawn from the widely used nomograms established in the 1990s. These are population reference values, not individual targets.

The counterintuitive part is the shape of the curve. Actual amniotic fluid volume rises from roughly 250 mL at 16 weeks to a peak of about 800 to 1,000 mL around 34 weeks, then declines to around 500 to 600 mL at 40 weeks. The index, however, stays almost flat. The reason is mechanical: as the fetus and uterus enlarge, the same volume gets distributed across a much larger cavity with more fetal body occupying space, so each pocket's depth stays roughly constant. The index is a ratio, not a volume.

This is the single most misunderstood point about the chart. A woman with a completely normal ultrasound at 38 weeks might be alarmed to see her index at 8 cm when her 28-week value was 17 cm, but 8 cm sits within normal at term. The chart's flatness is not a measurement error.

Oligohydramnios: Thresholds and Mechanism

Oligohydramnios is defined as an amniotic fluid index below 5 cm. Using the single deepest pocket method, the cutoff is below 2 cm. Both thresholds describe the same clinical state with different measurement tools, and a woman should not be told she has one of each based on the two numbers being different, because the units and methods differ. Per the American College of Obstetricians and Gynecologists, an index below 5 cm or a deepest pocket below 2 cm both qualify, and the two cutoffs are not interchangeable without naming the method.

The prevalence is estimated around 1 to 2 percent of pregnancies at term and higher in post-term pregnancies, where up to 10 percent or more may show reduced fluid. In the second trimester, severe oligohydramnios is far rarer but more consequential.

The mechanism is worth walking through because it explains why the condition matters. Amniotic fluid in the second half of pregnancy is overwhelmingly fetal urine. The fetus swallows fluid and produces urine, and the sac turns over its entire volume roughly every eight to twelve hours. Reduced fluid therefore points to one of four problems: the fetus is not producing urine, the fetus is not swallowing normally, the membranes are leaking, or the placenta is not delivering enough blood flow to support kidney output.

The placenta link is why oligohydramnios is treated as a signal of placental insufficiency. When placental perfusion falls, the fetus shunts blood to the brain and heart and away from the kidneys, which reduces urine output. The fluid drop appears before other signs of fetal compromise. This is why low fluid at term triggers closer surveillance even when the baby looks healthy on a non-stress test.

Polyhydramnios: High Fluid and Its Causes

Polyhydramnios is defined as an amniotic fluid index above 25 cm, or a deepest vertical pocket above 8 cm. Mild cases run from 25 to 29 cm, moderate from 30 to 34 cm, and severe from 35 cm and above. Prevalence is roughly 1 to 2 percent of pregnancies, with mild cases accounting for the vast majority.

The mechanism is the mirror image of oligohydramnios: excess fluid accumulates when fetal swallowing is impaired or urine output is high. The most common association is gestational diabetes, which drives fetal hyperglycemia and osmotic diuresis, increasing urine production. That accounts for a large share of mild cases. Fetal swallowing problems from esophageal atresia or neurological impairment are rarer causes. Between 50 and 60 percent of polyhydramnios cases have no identifiable cause.

Severe polyhydramnios, above 35 cm, deserves the most attention. It raises the risk of preterm labor from uterine overdistension, placental abruption when the membranes rupture, and postpartum hemorrhage. The practical point for a reader is that a mildly elevated index at 30 weeks often resolves with better glucose control and needs monitoring, not panic.

The Comparison That Changed Practice

The amniotic fluid index versus single deepest pocket debate is the most important technical comparison in this field, and it is not in most consumer-facing explainers. The two methods disagree often, and the disagreement has clinical consequences.

A 2012 systematic review by the Cochrane collaboration and a subsequent large trial found that using the single deepest pocket reduced the rate of unnecessary interventions, including inductions and cesarean deliveries, without increasing poor neonatal outcomes. The index flags more cases as abnormal that turn out fine. That is the trade-off: the four-quadrant sum catches more potential problems but also more false alarms.

The practical result is that many American practices have shifted to single deepest pocket for routine surveillance while retaining the index for specific protocols. If you are handed a number, confirm which method produced it before you compare it to any chart.

What the Chart Cannot Tell You

The chart has real limits that deserve explicit statement. First, it is a screening tool, not a diagnosis. A low or high value prompts further testing, not a treatment decision on its own. Second, inter-observer variability is significant: two experienced sonographers measuring the same woman on the same day can differ by 2 to 3 cm. A single borderline value is less meaningful than a trend.

Third, the index does not predict outcomes reliably on its own. Studies have found poor correlation between isolated low fluid at term and adverse neonatal outcomes when other tests are normal. This is why the American College of Obstetricians and Gynecologists and the Society for Maternal-Fetal Medicine have moved away from treating the index as a standalone trigger for delivery.

Fourth, the chart was built from a largely specific population in the 1990s. Body mass index, maternal hydration, and time of day all shift the reading. Maternal hydration alone can raise the index by 1 to 2 cm within hours, which is why some protocols recommend oral fluids before a repeat scan.

Red Flags That Override the Number

Certain findings matter more than the chart value itself and warrant immediate contact with a clinician. These are not substitutes for medical advice, but they are the situations where the number on the printout is not the whole story.

• Vaginal leaking of clear fluid, which suggests membrane rupture regardless of index value
• A sudden drop of more than 5 cm from a prior normal reading in the same trimester
• An index below 5 cm before 24 weeks, which carries a much graver prognosis than at term
• An index above 35 cm with rapid uterine growth or difficulty breathing
• Reduced fetal movement combined with any abnormal fluid reading
• Fever, abdominal pain, or uterine tenderness alongside a low reading, which raises concern for infection

What to Actually Do With Your Number

The practical actions are concrete. Ask which method produced the measurement, index or deepest pocket, and write it down. Ask for the gestational week at the time of the scan, because the same number means different things at 24 and 38 weeks. Request the actual value in centimeters rather than a verbal "low" or "normal."

If the value is borderline, ask whether a repeat scan in 24 to 48 hours is appropriate. Hydration status, position, and timing all shift the reading, and a second measurement often resolves the ambiguity. If the value is clearly abnormal, ask what the plan is for fetal surveillance, whether a non-stress test or biophysical profile is scheduled, and what would trigger delivery.

Keep a simple log of each measurement with its date, week, method, and value. Trends matter more than any single point, and a clinician reviewing three data points can make a better call than one working from a single isolated number. The chart is a reference, not a verdict, and the person holding it should know which column they are reading.