Normal Amniotic Fluid Index Size on Ultrasound >24 Weeks
The amniotic fluid index (AFI) is a sonographic measurement that quantifies amniotic fluid volume by summing the deepest vertical pocket in each of the four uterine quadrants. Accurate AFI assessment after 24 weeks is essential for monitoring fetal well-being, placental function, and identifying oligohydramnios or polyhydramnios in a timely manner.
Normal Reference Values
| Measurement |
|---|
| <5 cm |
Clinical Significance
An AFI of less than 5 cm after 24 weeks gestation defines oligohydramnios, a finding associated with increased perinatal morbidity and mortality. Reduced amniotic fluid may reflect uteroplacental insufficiency, fetal renal anomalies, or membrane rupture, and warrants prompt clinical evaluation and escalation of fetal surveillance.
Oligohydramnios is associated with umbilical cord compression, fetal growth restriction, meconium aspiration, and pulmonary hypoplasia in severe or early-onset cases. Clinicians should correlate AFI findings with clinical history, biophysical profile scores, and Doppler studies before making management decisions, as borderline values can be technique-dependent.
- Uteroplacental insufficiency — most common cause in the third trimester
- Fetal renal agenesis or obstructive uropathy — especially in early-onset oligohydramnios
- Premature rupture of membranes (PROM) — always exclude clinically
- Post-dates pregnancy — physiologic reduction beyond 41 weeks
- Maternal dehydration or NSAID use — reversible, often overlooked causes
Reference: Phelan JP, Smith CV, Broussard P et al. Amniotic fluid volume assessment with the four-quadrant technique at 36-42 weeks’ gestation. J Reprod Med. 1987;32(7):540-2.
Imaging Notes
The AFI is measured using transabdominal ultrasound with the transducer held perpendicular to the floor and parallel to the maternal sagittal plane. The uterus is divided into four quadrants using the umbilicus as the horizontal landmark and the linea nigra as the vertical landmark. The deepest vertical pocket free of fetal parts and umbilical cord is measured in each quadrant, and the four values are summed to yield the AFI in centimeters.
Care should be taken to avoid including cord loops within the measured pocket, as this falsely elevates the AFI. Color Doppler can help identify and exclude cord loops. Patient positioning in a slight left lateral decubitus tilt and adequate transducer pressure minimises measurement variability. The single deepest pocket (SDP) technique is an alternative method; an SDP of less than 2 cm also defines oligohydramnios and may have superior specificity in some clinical contexts.