or a gestational age-specific percentile reference range. he AFI is generally considered normal if greater than 5 cm and below 24 or 25 cm. Values outside these ranges indicate oligohydramnios and hydramnios, respectively. The upper threshold of 24 cm is used in consensus documents (American College of Obstetricians and Gynecologists, 2016; Reddy, 2014). The 25-cm threshold is oten applied in research studies (Khan, 2017; Luo, 2017; Pri-Paz, 2012). Moore and Cayle (1990) have provided normal curves for AFI values based on a cross-sectionl evaluation of nearly 800 uncomplicated pregnancies. he mean AFI was found to be between 12 and 15 cm from 16 weeks until 40 weeks' gestation. Other investigators have published nomograms with similar mean values (Hinh, 2005; Machado, 2007). Figure 11-1 depicts these AFI nomogram reference values in relation to commonly used thresholds for hydramnios and oligohydramnios.
This is an abnormally increased amnionic luid volume, and it complicates 1 to 2 percent of singleton pregnancies (Dashe, 2002; Khan, 2017; Pri-Paz, 2012). It is more frequently noted in multifetal gestations (Hernandez, 2012). Hydramnios may be suspected if the uterine size exceeds that expected for gestational age. The uterus may feel tense, and palpating fetal small parts or auscultating fetal heart tones may be diicult. An extreme example is shown in Figure 11-2.
Hydramnios may be urther categorized according to degree. Such categorization is primarily used in research studies to stratiy risks. Several groups have termed hydramnios as mid if the AFI is 25 to 29.9 cm; moderate, if 30 to 34.9 cm; and severe, if 35 cm or more (Lazebnik, 1999; Luo, 2016; Odibo, 2016; Pri-Paz, 2012). Mild hydramnios is the most common, comprising
As with the single deepest luid pocket measurement, the ultrasound transducer is held perpendicular to the loor and parallel to the long axis of the woman. The uterus is divided into four equal quadrants-the right and left upper and lower quadrants, respectively. The AFI is the sum of the single deepest pocket from each quadrant. The intraobserver variability of the AFI approximates 1 cm, and the interobserver variability is about 2 cm. Variations are larger when luid volumes are above the normal range (Moore, 1990; Rutherford, 1987). A useful guideline is that the AFI approximates three times the single deepest pocket of fluid (Hill, 2003).
Determination of whether the AFI is normal may be based on either a static numerical threshold
FIGURE 11-2 Severe hydramnios-5500 mL of amnionic fluid was measured at delivery.
97.5th percentile 2.5th percentile
FIGURE "-3 Sonogram of severe hydramnios at 35 weeks in a pregnancy complicated by fetal aqueductal stenosis. This pocket of amnionic fluid measureso> 15 cm, and the amnionic fluid index measured nearly 50 cm.