- Isolated term oligohydramnios, as defined by an amniotic fluid index (AFI) of less than 5 cm, has not been shown to be associated with poor maternal or fetal outcomes. Management may be individualized based on factors such as parity, cervical ripeness, and patient preference (SOR: B).
- Maternal hydration with oral water has been shown to increase AFI in a few hours, likely due to improved uteroplacental perfusion. This is a reasonable alternative to immediate labor induction in women with isolated term oligohydramnios (SOR: B).
- An isolated finding of a so-called “border-line” AFI (5–8 cm) is not an indication for labor induction (SOR: B).
Family physicians providing maternity care often face a scenario in which an otherwise low-risk, term patient is incidentally noted to have a low amniotic fluid index (AFI). Common reasons for obtaining an AFI in a woman with a low-risk pregnancy include evaluation of decreased fetal movement, spontaneous variable decelerations during monitoring to evaluate for labor, or an ultrasound evaluation for fundal height measurements discordant with gestational age. How should “isolated” oligohydramnios—an AFI <5 cm—be interpreted, and should immediate induction be recommended for such patients?
Oligohydramnios occurs in about 1% to 5% of pregnancies at term.1,2 Because adverse outcomes occur in high-risk pregnancies complicated by low amniotic fluid volume, oligohydramnios commonly prompts labor induction.1,3,4 At one university center, oligohydramnios is now the leading indication for labor induction.5 Many centers may even induce labor when the AFI is between 5 cm and 8 cm, the so-called borderline AFI.3
Labor induction increases the use of cesarean delivery, particularly for the primiparous woman with an unripe cervix.6 Recent studies questioning the safety of labor induction in women who have had a cesarean may increase the number of elective repeat cesarean procedures when delivery is believed indicated for oligohydramnios.7 (See Underlying causes of oligohydramnios.)
By the second trimester, amniotic fluid is being produced primarily through fetal urine production and is primarily resorbed through fetal swallowing. Significant amounts of amniotic fluid are also produced and resorbed by the fetal lung and directly resorbed from the amniotic cavity by the placenta.8,9 Amniotic fluid volume is affected by the status of maternal hydration and maternal plasma osmolality.10-13
Acute oligohydramnios may occur from ruptured membranes, usually diagnosed by clinical signs and vaginal fluid with altered pH and a ferning pattern on microscopic exam.
Chronic oligohydramnios arises from prerenal, renal, and postrenal causes. The latter 2 groups reflect fetal kidney and urogenital abnormalities, which directly decrease fetal amniotic fluid production. Uteroplacental insufficiency is the most common cause of prerenal oligohydramnios, and the decreased amniotic fluid is a direct result of decreased fetal renal perfusion.14 Uteroplacental insufficiency may result in intrauterine growth restriction as the fetus shunts blood away from the growing torso and limbs and to vital organs such as the brain. Preeclampsia and postdate pregnancies both involve pathologic changes in the placenta that may result in uteroplacental insufficiency and oligohydramnios.
Oligohydramnios is difficult to assess
True oligohydramnios can be difficult to confirm due to the questionable accuracy of amniotic fluid measurement by ultrasound. There is controversy, for example, about whether (and how) to include pockets of amniotic fluid containing umbilical cord.15 The AFI was introduced in 19872 to replace the 2 cm “pocket technique” of fluid assessment, and studies continue to question to what extent the AFI reflects actual amniotic fluid volume.
AFI measurements may vary with the amount of pressure applied to the abdomen and with fetal position or movement.16
Serial measurements taken by the same ultrasound operator have been shown to differ from the true volume by 1 cm, or 10.8%; serial measurements taken by multiple operators have differed by as much as 2 cm, or 15.4%.17,18
O’Reilly-Green compared the diagnosis of oligohydramnios in 449 post-term patients with actual amniotic fluid volume measured at rupture of membranes.19 They found a positive predictive value of 50% for oligohydramnios at an AFI of 5 cm as the lower limit of normal. A study of 144 third trimester patients using the dye-dilution technique found that, to achieve 95% confidence for ruling out oligohydramnios, a cutoff AFI of 30 cm would need to be used, a value consistent with polyhydramnios.20
What is the association between oligohydramnios and poor fetal outcomes?
A number of studies over the past 15 years have shown an association between oligohydramnios and poor fetal outcomes. These were predominantly retrospective studies, which failed to control for the presence of factors known to be associated with oligohydramnios such as intrauterine growth restriction (IUGR) and urogenital malformations.