Disorders of the Amniotic Fluid

Basics of Amniotic Fluid

Pregnant woman getting ultrasound scan in doctor's office
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Of all of the modern miracles of science, we still don't know where amniotic fluid really originates.

We know that the fluid after a certain point does contain fetal urine, but how can we explain it prior to the baby's ability to make urine?

Other amazing facts include that the amniotic fluid continually replaces itself at the rate of every three hours.

That said, we've been trying to define what is normal amniotic fluid and what is abnormal.

There are four categories of amniotic fluid:

  1. Oligohydramnios
  2. Pockets seen greater than 1 cm in diameter (normal)
  3. Adequate fluid, seen everywhere between the fetus and uterine wall (normal)
  4. Polyhydramnios

This measurement is commonly taken by using an ultrasound to determine the Amniotic Fluid Index (AFI). The most recent studies say that the AFI is not a great predictor of the Amniotic fluid volume (actual amount of fluid). In fact, another study confirmed these finding, for either extreme in fluid volume.

Oligohydramnios

When a woman is said to have too little amniotic fluid she has oligohydramnios. This is defined as having less than 200 ml of amniotic fluid at term or an AFI of less than 5 cm. This means that during an ultrasound the largest pocket of fluid found did not measure 1 cm or greater at its largest diameter. It is clinically very hard to prove prior to delivery. After the birth, examining the placenta for the presence of amnion nodosum on the placenta is highly correlated with oligohydramnios.

Depending on when the woman is diagnosed with oligohydramnios, there are different complications to look for, although the majority of women diagnosed will not have problems.

In early pregnancy there is the worry of amniotic adhesions causing deformities or constriction of the umbilical cord. There is also concern about pressure deformities, like clubfeet, from not having enough free space in the womb.

Even with oligohydramnios, ultrasound resolution and screening for anomalies is very adequate. So ultrasound is still an effective way to screen for deformities both associated and non-associated with the oligohydramnios.

Later in pregnancy oligohydramnios is one of the signs of fetal distress. This occurrence can cause compression of the cord, which can lead to fetal hypoxia, meaning that the baby is not getting enough oxygen.

Induction is not always the best option when oligohydramnios is present.

There are many factors that need to be taken into consideration.

Meconium, if passed cannot be diluted in cases of true oligohydramnios, however, one study found that there were fewer incidences of meconium staining when low amniotic fluid volumes were reported. However, there was an increase in the numbers of babies having fetal distress requiring a cesarean birth.

Other concerns with oligohydramnios:

  • Intrauterine Growth Retardation
  • Prolonged Rupture of Membranes
  • Fetal Malformations (Renal Agenesis, polycystic kidneys, urethral obstruction, etc.)
  • Postmaturity Syndrome

Diabetes is commonly thought of as a reason for oligohydramnios, it does not have to cause a problem with the pregnancy with proper treatment.

What treatment options are available for women with oligohydramnios?

Originally we felt that replacing the fluid through amnioinfusion was a great idea. However, this did not appear to be beneficial. We do know that immersion works well at reversing the signs of oligohydramnios.

In the absence of IUGR and fetal anomalies, women diagnosed with oligohydramnios can have an appropriately sized baby with no health problems.

Polyhydramnios

Polyhydramnios is the opposite end of the scale, being defined as 2000 ml of fluid at term or greater. This occurs in fewer than 1 % of the pregnancies.

While some feel that polyhydramnios is a cause for preterm labor because of uterine distension, polyhydramnios in and of itself is not a predictor for preterm labor, rather the cause of the increase in fluid is predictive of whether the pregnancy will go to term.

Polyhydramnios is more likely to occur when:

  • There is a multiple gestation.
  • There is Maternal diabetes.
  • There is a congenital malformation.

There are varying degrees of polyhydramnios. The severity of polyhydramnios does not have an influence on the weight of your baby, as earlier studies had predicted.

Treatment is varying for polyhydramnios, including drug treatments, selective use of amniocentesis to reduce the fluid volume.

Left untreated there may be further risks at the birth, small in number, but they should be addressed. This would include a greater incidence of cord prolapse, fetal malpresentation, placental abruption, and postpartum hemorrhage.

Considering that the current testing is not beneficial in all aspects of prediction, we need to address how to find a manner that is non-invasive to treat these disorders of amniotic fluid. So the question becomes how often do we test, who do we test, and what do we do with the results?

Right now, the answers are not clear and should be taken on a case by case basis.

They majority of women diagnosed with either of these problems, will not give birth to a baby with a problem, but the concern is there and does need to be appropriately addressed by her care provider.

Additional References:

Acute Obstetrics: A Practical Guide, Heppard and Garite, 1996, Mosby.
Human Labor and Birth, 5th Edition, Harry Oxorn, 1986, Prentice Hall.

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