Second- and third-trimester obstetrical ultrasound examinations include an amniotic fluid volume assessment. Professional organizations’ clinical guidance recommends using semiquantitative techniques, such as the single deepest vertical pocket or amniotic fluid index, for this purpose. The single deepest vertical pocket is described as the preferred method of assessing amniotic fluid volume based on fewer oligohydramnios diagnoses and labor inductions with no demonstrable difference in pregnancy outcomes compared with the amniotic fluid index. We offer an alternative interpretation of the evidence for this advice, drawn from 6 randomized clinical trials and 2 meta-analyses comparing the single deepest vertical pocket to the amniotic fluid index.
Individually and collectively, these reports are underpowered to detect significant differences in maternal and perinatal outcomes by study group. Moreover, randomized clinical trials comparing maternal and perinatal outcomes resulting from a policy of labor induction at or beyond 37 weeks of gestation vs expectant care consistently favor labor induction, the very intervention paradoxically cited as favoring the single deepest vertical pocket vs the amniotic fluid index. We conclude that the amniotic fluid index should be considered a reasonable method for third-trimester amniotic fluid assessment and diagnosing oligohydramnios.
Introduction
All second- and third-trimester ultrasound examinations require an amniotic fluid volume (AFV) assessment. , In addition, sonographic AFV evaluation is an integral part of antepartum fetal surveillance tests, such as the fetal biophysical profile (BPP) and modified BPP (nonstress test [NST] with amniotic fluid assessment). Although a subjective determination of AFV is reliable in the hands of experienced examiners, semiquantitative measurement techniques, such as the single deepest fluid pocket (SDP) and amniotic fluid index (AFI), were broadly integrated into clinical practice following their introduction over 3 decades ago. , However, clinical guidance universally expresses a clear preference for employing the SDP rather than the AFI. ,
The rationale for this recommendation is based on the findings of randomized clinical trials (RCTs) comparing the SDP to the AFI in evaluating AFV in singleton pregnancies. Taken individually and collectively, these studies uniformly demonstrate similar perinatal outcomes with both techniques, whereas the AFI incurs increased rates of diagnosing oligohydramnios and obstetrical interventions. Our review of the same evidence leads us to draw very different conclusions and suggest revising existing clinical recommendations. It is our clinical opinion that the AFI should be an acceptable option for AFV assessment, including diagnosing oligohydramnios. In addition, we contend that the very performance characteristics leading to recommending SDP vs AFI in diagnosing oligohydramnios may support the preferential use of AFI vs SDP in this regard.
Randomized Clinical Trials and Clinical Guidance
Of note, 6 published RCTs compare the AFI and the SDP as screening tests for preventing adverse pregnancy outcomes. The characteristics of the RCTs are presented in Table 1 . The first 5 studies were included in a 2008 Cochrane Library meta-analysis, with the findings of the planned primary and secondary outcomes presented in Table 2 . Although the overall cesarean delivery rate was not significantly different between the groups, the increased rate of diagnosing oligohydramnios, labor induction, and cesarean delivery for fetal distress coupled with the statistically similar rates of primary and secondary outcomes led the authors to conclude that “the SDP measurement seems to be the more appropriate method for assessing AFV during fetal surveillance.” Of note, 2 studies employed lower thresholds for diagnosing oligohydramnios using SDP and higher cutoffs for diagnosing oligohydramnios using AFI. , These important methodological differences must be considered as potential contributors to the increased frequency of diagnosing oligohydramnios and thus intervention when using the AFI compared with the SDP. The authors further noted the need for a systematic review of the diagnostic accuracy of the AFI vs the SDP and additional trials to create outcomes-based consensus in standardizing the method of diagnosing decreased amniotic fluid and timing and mode of delivery in affected pregnancies. This caveat is reasonable, considering that a total of 500 subjects demonstrated oligohydramnios by either AFI or SDP measurements.
| Study | Setting | Inclusive years | Subjects (all singleton pregnancies) | Subjects assigned to each group (n) | Oligohydramnios definition | Oligohydramnios frequency, n (%) | Primary outcome | Additional outcomes | Fetal testing | Intervention | ||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| SDP | AFI | SDP | AFI | SDP | AFI | P value | ||||||||
| Alfirevic et al 1997 | United Kingdom | 1994–1995 |
| 250 | 250 | <1.8 cm | <7.3 cm | 6 (2.4) | 25 (10) | .0008 | Cesarean delivery |
| AFV and CTG | Testing every 3 d with labor induction for abnormal CTG or AFV, 43 wk, maternal request, developed exclusion criterion |
| Oral et al 1999 | Turkey | 1997–1998 |
| 53 | 48 | <1.8 cm | <5.5 cm | 2 (3.8) | 8 (16.7) | .04 | Not specified |
| AFV and CTG | Twice weekly testing with delivery for abnormal CTG, oligohydramnios, or 42 wk |
| Moses et al 2004 | United States | 2001–2003 |
| 501 | 499 | <2 cm | ≤5.0 cm | 42 (8.4) | 124 (24.8) | <.001 | Cesarean delivery for fetal distress |
| AFI or SDP | Assessment of amniotic fluid by assigned method Results not used for management |
| Magann et al 2004 | United States | Not reported |
| 264 | 273 | <2 cm | ≤5.0 cm | 46 (17.4) | 102 (37.4) | <.001 | Cesarean delivery for fetal distress |
| BPP using AFI or BPP using SDP | Delivery at 34 wk for premature rupture of membranes or oligohydramnios or at time of diagnosis if later in pregnancy |
| Chauhan et al 2004 | United States | 1997–2001 |
| 558 | 530 | <2 cm | ≤5.0 cm | 57 (10.2) | 88 (16.6) | .002 | Cesarean delivery for nonreassuring fetal heart rate tracing |
| NST and AFI or NST and SDP | Assessment of AFV by assigned method Results known to managing clinicians |
| Kehl et al 2016 | Germany | 2012–2013 |
| 504 | 498 | <2 cm | ≤5.0 cm | 11 (2.2) | 49 (9.8) | <.01 | Postpartum admission to the NICU |
| AFI or SDP | Labor induction for oligohydramnios, discharge home if normal fluid and not in labor with next fluid assessment at next admission if >7 d later |
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