401.9
Amniotic fluid volumes are in mL.
Source: Sandlin et al. 2014.37 Reproduced with permission of Springer Business + Science Media.
Polyhydramnios occurs in 1–2 percent of all pregnancies35 and is associated with fetal malformations in about 40 percent of cases.40 Moise defined polyhydramnios in singleton or twin pregnancies as MVP >8 cm in the late second and the third trimesters, and oligohydramnios as MVP <2 cm.41 Most common are neural tube defects (NTDs) and disorders that impair deglutition or absorption of AF (esophageal and other intestinal atresias or obstructions).26, 40, 42, 43 Myotonic dystrophy was identified in 4 of 41 patients with “idiopathic” polyhydramnios.44 The development of polyhydramnios in anencephaly is believed to result from impaired swallowing by the fetus43 or deficient antidiuretic hormone production by the fetus.45 Polyhydramnios also occurs in maternal diabetes mellitus and may appear, especially early in pregnancy, in monozygotic twins.43 Irrespective of the cause, there seems to be a risk of recurrence for polyhydramnios of between 0.06 and 8 percent.42, 43
Oligohydramnios, in contrast, is most often associated with disorders of the urinary tract that interfere with micturition, such as renal agenesis.46 Placental insufficiency and extrauterine pregnancy also may cause oligohydramnios, as can exposure to maternal hypertension treatment with renin–angiotensin system blockers.47 Sherer48 cautioned that decreased AFV is especially of concern when it occurs in conjunction with structural fetal anomalies, fetal growth restriction, or maternal disease, and others have noted an increased risk of fetal heart rate abnormalities.49 However, a normal AFV may occur even in the presence of urinary tract obstruction or bilateral renal agenesis.26 A normal AFV was found in 13 cases of hydronephrosis, eight of which had confirmed urinary tract obstruction at birth; the other five were normal.50
