Normal Range for Oligohydramnios
The term "normal range for oligohydramnios" is contradictory—oligohydramnios itself represents an abnormal condition defined as AFI < 5 cm or MVP < 2 cm, not a normal range. 1, 2, 3
Defining Amniotic Fluid Volume Categories
Normal amniotic fluid volume is defined as:
- AFI: 5-24 cm (with values >8 cm generally considered reassuringly normal) 3
- MVP: ≥ 2 cm throughout gestation 3
Oligohydramnios (abnormally low fluid) is defined as:
Borderline/low-normal amniotic fluid:
Clinical Significance of These Thresholds
Oligohydramnios (AFI < 5 cm) carries significant risks:
- 2.6-fold increased risk of stillbirth (OR 2.6; 95% CI 2.1-3.2) 1, 2
- Higher rates of intrapartum fetal heart rate abnormalities and cesarean delivery for fetal distress 1
- 25% rate of major fetal malformations in preterm pregnancies 4
Borderline AFI (5-8 cm) also carries elevated risks:
- 10% rate of major malformations and 21% rate of growth restriction below 3rd percentile in preterm pregnancies 4
- 16.2% chance of progressing to oligohydramnios within 4 days, warranting twice-weekly surveillance 5
MVP vs AFI: Important Clinical Distinction
MVP measurement is preferred over AFI for clinical decision-making because:
- AFI diagnoses oligohydramnios more frequently than MVP, leading to more obstetric interventions without improving perinatal outcomes 1, 6
- The SAFE trial demonstrated that AFI increased labor induction rates 3.5-fold compared to SDP/MVP without reducing NICU admissions 6
- Neither method performs better at preventing intrauterine fetal demise, but MVP reduces unnecessary interventions 1, 3
Common Pitfall to Avoid
Do not confuse "oligohydramnios" with a normal range—the question as phrased contains a conceptual error. Oligohydramnios is the diagnosis of abnormally low fluid, not a range within normal. The correct framework is: normal fluid (AFI ≥5 cm or MVP ≥2 cm) versus oligohydramnios (AFI <5 cm or MVP <2 cm). 1, 2, 3