Normal Single Deepest Vertical Pocket at 28 Weeks Gestation
At 28 weeks gestation in a healthy singleton pregnancy, the normal single deepest vertical pocket (also called maximum vertical pocket or MVP) should measure ≥2 cm, with values below 2 cm defining oligohydramnios. 1
Normal Reference Values
The single deepest pocket measurement of ≥2 cm is considered normal throughout gestation, including at 28 weeks, and this threshold remains constant across all gestational ages. 2
In a large prospective study of normal pregnancies, the mean single deepest pocket at 28 weeks was approximately 5-6 cm, with the 5th percentile around 3 cm and the 95th percentile around 8 cm. 3
Contemporary U.S. data from the NICHD Fetal Growth Studies show that single deepest pocket values peak around 33 weeks gestation, with the 3rd percentile remaining well above 2 cm at 28 weeks (approximately 3-4 cm). 4
Clinical Significance of the Single Deepest Pocket
The single deepest pocket is the preferred method for assessing amniotic fluid volume because it results in fewer false-positive diagnoses of oligohydramnios compared to the amniotic fluid index (AFI), leading to fewer unnecessary interventions without compromising detection of adverse outcomes. 1, 5
A Cochrane systematic review demonstrated that using the single deepest pocket instead of AFI reduces the diagnosis of oligohydramnios by more than half (RR 2.33 for AFI diagnosing more cases) and decreases induction of labor rates (RR 2.10) without any difference in perinatal outcomes including NICU admission, low Apgar scores, or umbilical artery pH. 5
The single deepest pocket has only a 1% false-positive rate for oligohydramnios compared to 8% with AFI and 30% with the two-diameter pocket method. 3
Pathologic Thresholds
Oligohydramnios is defined as a single deepest pocket <2 cm and is independently associated with a 2.6-fold increased risk of stillbirth (OR 2.6,95% CI 2.1-3.2). 1, 2
Polyhydramnios is defined as a single deepest pocket >8 cm in singleton pregnancies, though this threshold is more commonly applied in the context of twin-twin transfusion syndrome. 6
Common Pitfalls to Avoid
Do not use AFI as the primary measurement tool at 28 weeks, as it leads to overdiagnosis of oligohydramnios and unnecessary interventions without improving outcomes. 1, 5
Avoid diagnosing oligohydramnios based on clinical fundal height measurements alone when ultrasound documentation shows a normal single deepest pocket measurement. 7
Do not apply a single cutoff value across all gestational ages without considering that normal ranges vary slightly with gestational age, though the 2 cm threshold for oligohydramnios remains constant. 4
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