Normal Amniotic Fluid Index Size on Ultrasound
The amniotic fluid index (AFI) is a standardized sonographic measurement used to quantify amniotic fluid volume during pregnancy. It is calculated by summing the deepest vertical pocket from four uterine quadrants, with the transducer held perpendicular to the floor. Accurate AFI assessment is essential for identifying polyhydramnios, a finding associated with fetal, placental, and maternal complications.
Normal Reference Values
| Measurement |
|---|
| >25 cm |
Clinical Significance
Polyhydramnios is defined as an AFI greater than 25 cm on ultrasound. The severity of elevation correlates with perinatal risk; higher maximal AFI values have been shown to carry prognostic significance in affected pregnancies. Mild polyhydramnios (AFI 25–29.9 cm) is most commonly idiopathic, whereas moderate-to-severe cases (AFI ≥30 cm) warrant thorough investigation for an underlying etiology.
Complications associated with polyhydramnios include preterm labor, premature rupture of membranes, placental abruption, fetal malpresentation, and umbilical cord prolapse. Perinatal mortality risk increases proportionally with the degree of fluid excess. Pitfalls include over-estimation of AFI in obese patients and misidentification of fetal small parts or loops of cord as fluid pockets.
- Idiopathic polyhydramnios – most common cause, diagnosis of exclusion
- Fetal gastrointestinal obstruction – e.g., duodenal atresia, esophageal atresia
- Fetal neuromuscular disorders – impaired swallowing reflex
- Maternal diabetes mellitus – fetal polyuria secondary to hyperglycemia
- Fetal anemia or hydrops – isoimmunization or parvovirus B19 infection
Reference: Pri-paz S, Khalek N, Fuchs KM et al. Maximal amniotic fluid index as a prognostic factor in pregnancies complicated by polyhydramnios. Ultrasound Obstet Gynecol. 2012;39(6):648-53.
Imaging Notes
AFI is measured using real-time B-mode ultrasound with the patient in the supine position. The uterus is divided into four quadrants by the linea nigra (vertical) and the umbilicus (horizontal). In each quadrant, the transducer is oriented perpendicular to the floor and parallel to the maternal spine; the deepest vertical pocket free of fetal parts and umbilical cord is measured. The four values are summed to yield the AFI. Gain settings should be optimized to distinguish fluid from soft tissue, and color Doppler can help exclude cord within a suspected pocket.
The single deepest pocket (SDP) technique is an alternative measurement method; an SDP greater than 8 cm corresponds broadly to polyhydramnios. AFI tends to show greater inter-observer variability than SDP, particularly at extremes of gestational age or in oligohydramnios. Serial AFI measurements are valuable for monitoring disease progression and response to intervention such as amnioreduction.