The Pseudo-Oligohydramnios/Hidden Pocket Study: The Added Contribution of Transvaginal Sonographic Evaluation of Amniotic Fluid Index

Background Normal amniotic fluid volume varies across gestation (Ounpraseuth 2017). Abnormal amniotic fluid volume has been associated with a variety of adverse pregnancy outcomes (Shrem 2016). In clinical practice, the amniotic fluid volume is a component of the biophysical profile which provides information on fetal well-being.

Transabdominal ultrasound is used to assess quantitatively the amniotic fluid volume, most commonly by either the Amniotic Fluid Index (AFI) or the Maximum Vertical Pocket (MVP) methods.

The value of the AFI method has been questioned in several studies, which have shown that an abnormal test, low or high, is neither highly accurate nor predictive of adverse outcome. Many pregnancies with normal AFI will be falsely characterized as abnormal, and a large number with truly abnormal AFI will be missed (Magann 1994, Chauhan 1997).

The MVP measurement may be preferred over the AFI method. A 2008 systematic review of randomized trials found that use of the AFI increased the rate of diagnosis of oligohydramnios (RR 2.3), induction of labor (RR 2.1), and cesarean delivery for fetal distress (RR 1.5) (Nabhan 2008).

Rationale and Hypothesis of the Study Recently, our group observed that when transvaginal ultrasound is being performed to non-laboring pregnant women at term for other obstetrical indications, there is a large amniotic fluid pocket above the internal cervical os and below the fetal presenting part. Due to the unique location of this pocket, we hypothesized that transvaginal ultrasound may provide better information than transabdominal ultrasound on amniotic fluid volume, particularly in cases when suspecting abnormal low amniotic fluid volume.