Oligohydramnios

Oligohydramnios (too little amniotic fluid) is best defined as an amniotic fluid index (AFI) <5th percentile, because normal amniotic fluid volume changes with gestational age.¹ Over the years it has been described as:

• Diminished amniotic fluid volume.
• Amniotic fluid volume of less than 500 mL at 32-36 weeks' gestation.
• Maximum vertical pocket (MVP) of less than 2 cm.
• Amniotic fluid index (AFI) of less than 5 cm, or less than the 5th percentile. This is an AFI of <6.8 cm at term.

Amniotic fluid volume (AFV) increases throughout most of pregnancy, with a volume of about 30 mL at 10 weeks gestation and a peak of about 1L at 34-36 weeks gestation. AFV decreases towards term, with a mean AFV of 800 mL at 40 weeks.²Amniotic fluid constantly circulates, with an estimated exchange rate as high as 3600 mL/h. Excretion of urine by the fetus is the major source of amniotic fluid production in the second half of the pregnancy. Fluid secreted by the fetal respiratory tract also contributes to AVF.

Fetal swallowing is the major pathway of amniotic fluid clearance in the last half of gestation. Fetal skin is highly permeable in the first half of pregnancy, but becomes keratinised at 22-25 weeks gestation, significantly reducing transfer.
The mother's fluid balance (and also therefore the fetus') have a major effect on the AFV. Increased maternal fluid intake has been shown to increase the AFV in women with oligohydramnios.

Oligohydramnios is secondary to either an excess loss of fluid, or a decrease in fetal urine production or excretion. Oligohydramnios is usually associated with one of the following conditions:

• Rupture of amniotic membranes (ROM).
• Congenital absence of functional renal tissue or obstructive uropathy.
  • Conditions that prevent the formation of urine or the entry of urine into the amniotic sac.
  • Fetal urinary tract malformations, including renal agenesis, cystic dysplasia, and ureteral atresia.
• Decreased renal perfusion leading to reduced urine production.
  • As a sequela of hypoxaemia-induced redistribution of fetal cardiac output.
  • In growth-restricted fetuses, chronic hypoxia results in shunting of fetal blood away from the kidneys to more vital organs.
  • Anuria and oliguria lead to oligohydramnios.
• Post-term gestation
  • The cause of decreased AFV in post-term pregnancies is unknown.
  • The decreased efficiency of placental function has been proposed as a cause, but this has never been confirmed histologically.
  • Decreased fetal renal blood flow and decreased fetal urine production have been demonstrated beyond 42 weeks in pregnancies involving oligohydramnios.

Oligohydramnios is a complication in approximately 4.5% of all pregnancies, and severe oligohydramnios is a complication in 0.7% of pregnancies.³Oligohydramnios is more common in pregnancies beyond term, because the AFV normally decreases at term. It complicates as many as 12% of pregnancies that last beyond 41 weeks.

Test for systemic lupus erythematosus (causes immune-mediated infarcts in the placenta and placental insufficiency). Other maternal risk factors (including hypertension and diabetes) should also be assessed.

Ultrasound

The diagnosis is confirmed by ultrasound. It may be discovered incidentally during routine scanning, or noted during antepartum surveillance for other conditions.

• Suspicion of oligohydramnios may be prompted by discrepancies in sequential fundal height measurements, or by fetal parts that are easily palpated through the maternal abdomen.
• During ultrasound normal-appearing fetal kidneys and fluid-filled bladder may be observed to rule out renal agenesis, cystic dysplasia, and ureteral obstruction.
• Fetal growth should be checked to exclude intrauterine growth restriction (IUGR) leading to oliguria. Doppler ultrasound could be used to assess placental insufficiency, if suspected.

Measurement of amniotic fluid volume

• The 2 most commonly used objective methods of determining AFV include measurement of the MVP depth and the summation of the depths of the largest vertical pocket in each quadrant, or the AFI. (The pregnant abdomen is divided into 4 quadrants by using the umbilicus as a reference point to divide the uterus into upper and lower halves, and by using the linea nigra to divide the uterus into left and right halves.) The 4 measurements are summed to obtain the AFI in centimetres.
• Pockets should be free of fetal limbs and the umbilical cord, although some allow for a single loop of cord to be within the fluid pocket. AFV may be artificially increased if the transducer is not maintained perpendicular to the floor. Excessive pressure on the maternal abdomen with the transducer may lead to an artificially reduced measurement.
• Both have equal diagnostic accuracy. The MVP technique may be a better means of assessing the AFV in twin gestations and in pregnancies at an early gestational age. Some study results have shown that the AFI has greater sensitivity and a higher predictive value than the MVP in diagnosing abnormally high and low AFVs.⁴ A Cochrane review has concluded that the MVP measurement during fetal surveillance seems a better choice since the use of the amniotic fluid index increases the rate of diagnosis of oligohydramnios and the rate of induction of labour without improvement in peripartum outcomes.⁵

Sterile speculum examination

• Sterile speculum examination should be performed to check for ROM. Amniotic fluid may pool in the vagina and a ferning pattern may be observed when fluid from the posterior vault is dried and examined under a microscope.
• Cervical mucus may cause false-positive results (as can semen and blood).
• Nitrazine paper/sticks turn blue. (The amniotic fluid is more basic - pH 6.5-7.0 - than normal vaginal discharge - pH 4.5)

Management of oligohydramnios is based on gestational age. Transfer to a tertiary referral centre may be appropriate if oligohydramnios is severe.

• Before term:
  • Expectant management is often the most appropriate course of action, depending on maternal and fetal condition.
  • Ongoing antepartum surveillance (including assessment of fetal growth and follow-up monitoring of AFV) is necessary.
  • Continuous fetal heart rate monitoring during labour has been advocated for all pregnancies complicated by oligohydramnios.
• At term:
  • Delivery is often the most appropriate management.
  • With reassuring fetal testing, delivery may be safely delayed on the basis of the parity, the gestational age, the inducibility of the mother's cervix and the severity of the oligohydramnios.
• After term:
  • Oligohydramnios in the post-term patient is associated with more fetal decelerations, a higher incidence of meconium-stained fluid and an increased risk for caesarean delivery.⁶
  • Oligohydramnios is considered an indication for delivery in a post-term pregnancy.
  • Some studies have shown increased perinatal mortality rates.^7,8

The treatment of maternal dehydration with oral or intravenous rehydration has been shown to increase the AFV by 30%.^3,9

Amnioinfusion

Increasing the amount of fluid within the amniotic cavity can be accomplished during delivery with the use of amnioinfusion:

• Warm or room-temperature sodium chloride solution is transcervically infused through an intrauterine catheter.
• This procedure increases the amount of fluid to provide more padding around the umbilical cord, which has been shown to decrease the frequency and severity of variable decelerations secondary to decreased cord compression.¹⁰
• The Royal College of Obstetricians and Gynaecologists does not recommend amnioinfusion for women with preterm rupture of membranes. Some have advocated the injection of fluid before delivery via amniocentesis to improve the intrauterine environment for the fetus prior to labour. The duration of the effect is unknown. Oligohydramnios is usually observed to return within 1 week.

Vesicoamniotic shunts

Vesicoamniotic shunts may be used to divert fetal urine to the amniotic fluid cavity in patients in whom a fetal obstructive uropathy is determined to be the cause of oligohydramnios. Although it is effective in reversing oligohydramnios, its ability to achieve sustainable good renal function in infancy is variable.¹¹ Pulmonary function cannot be guaranteed with restoration of the amniotic fluid volume.

• The earlier in pregnancy that oligohydramnios occurs, the poorer the prognosis. Fetal mortality rates as high as 80-90% have been reported with oligohydramnios diagnosed in the second trimester. Most of this mortality is a result of major congenital malformations and pulmonary hypoplasia secondary to PROM before 22 weeks' gestation. Midtrimester PROM often leads to pulmonary hypoplasia, fetal compression syndrome and amniotic band syndrome. The inspiration of amniotic fluid at regular intervals is probably needed for terminal alveolar development.
• AFV is a predictor of the fetal tolerance of labour, and it is associated with an increased risk of abnormal heart rate, meconium-stained amniotic fluid and caesarean delivery. An increased incidence of cord compression is associated with oligohydramnios; this can lead to variable decelerations.
• The assessment of AFV is important in pregnancies complicated by abnormal fetal growth or IUGR. Oligohydramnios is a frequent finding in pregnancies involving IUGR and is most likely secondary to decreased fetal blood volume, renal blood flow and subsequently fetal urine output. Pregnancies complicated by severe oligohydramnios have been shown to be at increased risk for fetal morbidity.⁸ In renal agenesis the mortality rate is 100%.¹²

• Pulmonary hypoplasia
• Fetal compression syndrome
• Amniotic band syndrome
• The risk of fetal infection is increased (by the presence of prolonged rupture of the membranes)