In routine obstetric practice, is the single deepest vertical pocket or the amniotic fluid index preferred for assessing amniotic fluid volume?

Single Deepest Vertical Pocket (MVP) is Preferred Over Amniotic Fluid Index

The single deepest vertical pocket (MVP) should be used as the primary method for assessing amniotic fluid volume in routine obstetric practice, rather than the amniotic fluid index (AFI). 1, 2

Why MVP is Superior to AFI

The American College of Radiology and contemporary guidelines recommend MVP because it:

• Reduces false-positive oligohydramnios diagnoses by approximately 50% compared to AFI, without compromising detection of adverse perinatal outcomes 2
• Decreases unnecessary obstetric interventions, including labor inductions and cesarean deliveries for fetal distress 2, 3
• Results in significantly fewer oligohydramnios diagnoses (AFI diagnoses 2.33 times more cases) and fewer labor inductions (2.10 times more with AFI) without any improvement in peripartum outcomes 3

Evidence Supporting MVP

The Cochrane systematic review of 4 randomized trials involving 3,125 women demonstrated that:

• Neither method prevents poor peripartum outcomes better than the other (NICU admission, umbilical artery pH <7.1, meconium, low Apgar scores) 3
• AFI leads to significantly more labor inductions (RR 2.10) and cesarean deliveries for fetal distress (RR 1.45) without improving outcomes 3
• The single deepest vertical pocket is the better choice since it avoids overdiagnosis and unnecessary interventions 3

Diagnostic Thresholds for MVP

• Normal MVP: ≥ 2 cm at any gestational age 1, 2
• Oligohydramnios: MVP < 2 cm (associated with 2.6-fold increased stillbirth risk) 1
• Polyhydramnios: MVP > 8 cm in singleton pregnancies 1, 2

Proper MVP Measurement Technique

The American College of Radiology specifies:

• Locate the single deepest vertical pocket anywhere in the uterus by systematically scanning all uterine regions 2
• Position the transabdominal probe perpendicular to the uterine contour and parallel to the maternal sagittal plane 2
• Record the maximal vertical depth in centimeters only (horizontal dimensions excluded) 2
• Exclude umbilical cord and fetal parts from the measurement 2

Common Pitfalls to Avoid

• Do not use AFI as the primary measurement tool, as it leads to overdiagnosis of oligohydramnios and unnecessary interventions without improving outcomes 1, 2
• Do not diagnose oligohydramnios based solely on clinical fundal-height measurements when ultrasound-derived MVP is within normal limits 1, 2
• When umbilical cord is present in the pocket, measure to the cord rather than through it for more accurate low-volume detection 4

Addressing Contradictory Evidence

While one 2022 commentary 5 argues that AFI remains reasonable based on underpowered studies, this interpretation contradicts:

• The consistent guideline recommendations from the American College of Radiology 2
• The Cochrane systematic review showing clear harm from AFI's overdiagnosis 3
• The established principle that avoiding unnecessary interventions without compromising outcomes is clinically superior 1, 2

The weight of guideline-level evidence and systematic review data strongly favors MVP over AFI for routine amniotic fluid assessment.