Polyhydramnios

Synonym: hydramnios

What is polyhydramnios?

Polyhydramnios, or amniotic fluid disorder, is defined as an abnormally large volume of amniotic fluid that surrounds your baby. It is known to be associated with adverse pregnancy outcomes and may raise suspicion of a problem with the pregnancy. Greater deviations from the norm are more strongly associated with abnormality.

Pathogenesis¹

Physiologically, the volume of amniotic fluid increases with gestation to a maximum at 36-37 weeks. It has a number of purposes, including protecting the fetus from trauma and infection, allowing lung development and facilitating the development and movement of the limb and other skeletal parts.

Mechanisms affecting amniotic fluid volume include:

• Fetal swallowing. By term, the fetus swallows 210-760 ml of amniotic fluid per day. Conditions which cause a problem with swallowing are a common cause of polyhydramnios - for example, atresia of the upper gastrointestinal tract, fetal hypoxia, neuromuscular disorders or brain abnormalities.

• Fetal urination. This is the main source of amniotic fluid in the second half of pregnancy - there is almost no amniotic fluid in cases of renal agenesis or fetal urinary tract obstruction. The fetus responds to changes by adjusting the amount of urination, thereby contributing to amniotic volume regulation. By term, urinary output is around 700-900 ml per day. Increases in fetal urination - for example, due to fetal anaemia and the subsequent increased cardiac output - may be a cause of increased amniotic fluid volume.

• Secretions of fetal lung fluid and fetal oral and nasal cavities.

• Absorption via intramembranous and transmembranous pathways. This includes movement of fluid between the fetal blood and placenta and across intrauterine membranes such as the amnion and chorion.

Conditions which affect any of these mechanisms may result in abnormal quantities of amniotic fluid; either polyhydramnios (too much) or oligohydramnios (too little).

Polyhydramnios causes²

Polyhydramnios causes are numerous and include:

• Idiopathic (the most common; no cause identified - around 50% of cases).

• Congenital anomalies and genetic disorders. 8-24% of cases. Anomalies may be isolated or due to a genetic disorder (see below). Includes conditions such as:

  • Oesophageal or duodenal atresia³ .

  • Cardiovascular defects.

  • Microcephaly or anencephaly.

  • Neural tube defects.

  • Renal defects, including Bartter's syndrome (a rare inherited defect in the loop of Henle of the renal tubule)⁴ .

• Genetic disorders as referred to above include:

  • Trisomy 21, 18 and 13.

  • Pena-Shokeir syndrome (contractures of the joints (arthrogryposis), growth problems, underdeveloped lungs, facial deformities).

  • Beckwith-Wiedemann syndrome (overgrowth disorder).

• Maternal diabetes. 5-26% of cases.

• Multiple pregnancy. 8-10% of cases.

• Fetal anaemia. 1-11% of cases.

• Congenital infections (eg, toxoplasmosis, parvovirus, rubella, cytomegalovirus).

• Hydrops fetalis (excessive amniotic fluid in one or more fetal compartment - eg, the pleural or abdominal space - common in rhesus haemolytic disease).

• Maternal substance misuse.

• Maternal metabolic abnormalities such as hypercalcaemia⁵ .

Polyhydramnios epidemiology

Polyhydramnios affects up to 1.6% of all pregnancies and this is generally similar across low- and middle-income countries⁶ . Rates are much higher however in pregnancies of women with diabetes (8.5%⁷ ) or gestational diabetes.

Where polyhydramnios is mild, an underlying abnormality is found in only 17%, whereas in moderate-to-severe polyhydramnios, this figure rises to 91%. There is also an association with increasing maternal age and with fetal macrosomia⁸ .

Polyhydramnios symptoms²

Polyhydramnios is suspected when antenatal examination reveals a uterus that is large for dates. Fetal parts may be difficult to palpate. It may be picked up on clinical or ultrasound examination.

When not diagnosed on routine antenatal examination, it may present with excessive maternal breathlessness, early onset of labour or rupture of membranes, umbilical cord prolapse, or abnormal fetal presentation.

Occasionally, the uterus enlarges rapidly. This is known as acute polyhydramnios and is most common in twin pregnancies. In such cases, abnormal connecting blood vessels in the twin placenta result in unequal distribution of blood flow (twin-to-twin transfusion syndrome)⁹ . The twin receiving the larger amount of blood supply is known as the recipient twin and the twin receiving the smaller amount is known as the donor. The recipient twin produces a large amount of urine and is surrounded by excessive amniotic fluid.

Differential diagnosis

When the uterus appears large for dates on clinical examination, ultrasound scans usually rule out the alternative diagnoses including:

• Multiple pregnancy.

• Fetal macrosomia.

• Abruptio placenta which may cause rapidly expanding uterine size due to the development of intrauterine haematoma. This is usually an easy differential diagnosis to make, as pain is a predominant feature.

• Chorioangioma - this is a benign lesion of the placenta due to excess capillary formation in the absence of villus differentiation. It may cause a 'large-for-dates' uterus per se, or be associated with polyhydramnios.

Investigations

Ultrasound¹

Experienced operators can detect polyhydramnios subjectively.

There are two methods used to quantify amniotic fluid volume:

• Amniotic fluid index (AFI), or four-quadrant method: the uterine cavity is divided into four quadrants or pockets. The largest vertical pocket in each quadrant is measured in centimetres and the total volume is calculated by adding the four together. A total of more than 24 cm defines polyhydramnios.

• Single deepest pocket (SDP) method: the deepest pocket is measured vertically. A measurement under 2 cm defines oligohydramnios and where it is over 8 cm there is polyhydramnios.

By both methods, the measurement can then further define polyhydramnios as mild, moderate or severe. The higher the measurement, the higher the likelihood of fetal abnormality¹⁰ .

Laboratory tests

The following may be helpful in excluding associated diseases:

• Blood glucose and oral glucose tolerance test.

• Maternal infection screen.

• If fetal anaemia or hydrops fetalis is suspected the following may also be appropriate:
  

  • Screening for maternal antibodies against fetal red blood cells.

  • Screening for cytomegalovirus, syphilis, rubella, toxoplasmosis, parvovirus 19.

  • Genotyping.

Amniocentesis and fetal karyotyping may be considered.

Polyhydramnios treatment and management^{1 11}

• Management is undertaken in secondary care. There are no guidelines or large studies to guide management decisions.

• The first step is to identify if there is an underlying cause.

• Identified causes are treated as appropriate. Fetal hydrops anaemia is treated with intravascular transfusion. If gestational diabetes is diagnosed, tight glycaemic control should be maintained. This may involve dietary manipulation, oral medication or insulin¹² .

• Mild polyhydramnios can be simply monitored and treated conservatively.

• Preterm labour is common due to overdistension of the uterus, and measures should be taken to minimise this complication. This includes regular antenatal checks and inspection of the uterus. Serial ultrasound scans should be carried out to monitor the AFI and fetal growth.

• Induction of labour should be considered if fetal distress develops. Induction by artificial rupture of the membranes (ARM) should be controlled, performed by an obstetrician and with consent to proceed to lower-segment caesarean section if required.

• Corticosteroids should be given to the mother antenatally if preterm delivery is imminent or considered¹³ . This helps to improve lung maturity.

• Prostaglandin synthetase inhibitors, particularly indometacin, may be used. This acts at least partly by reducing renal blood flow and reducing fetal urination. It is used usually for a maximum of 48 hours and regimes vary. It is not used in twin-to-twin syndrome or after 32 weeks, as adverse effects outweigh benefits in these cases. The main risk is fetal ductus arteriosus constriction and the risk increases with gestation. Sulindac has also been used but there are no trials to confirm efficacy and safety.

• Amnioreduction (drainage of amniotic fluid under ultrasound guidance) is also used in cases where indometacin is contra-indicated, in severe polyhydramnios, or in patients who are symptomatic. It is more commonly used in twin-to-twin transfusion syndrome. There are few studies but those that exist suggest it is safe and effective, although serial procedures may be required¹⁴ .

• Polyhydramnios associated with twin-to-twin syndrome may also benefit from laser ablation of the connecting placental vessels⁹ .

Polyhydramnios complications^{2 6}

There is a higher incidence of preterm labour and delivery. Other complications include premature rupture of the membranes, placental abruption (also reported as a consequence of amnioreduction¹⁵ ), malpresentation, postpartum haemorrhage and cord prolapse. There is a higher incidence of caesarean section.

For the mother, the risk of urinary tract infections is increased due to increased pressure on the urinary tract. The mother may have increased dyspnoea due to increased pressure on the diaphragm. There is also a higher risk of hypertension in pregnancy.

Studies of pregnancies associated with polyhydramnios but not congenital malformation showed that polyhydramnios was an independent risk factor for low birth weight, low Apgar scores, fetal death and neonatal mortality^{16 17} .

Prognosis

• Only 17% of pregnancies with mild polyhydramnios have a significant associated problem. However, an underlying abnormality is associated with 91% of cases of moderate-to-severe polyhydramnios¹¹ .

• A poor prognosis is associated with the more severe fetal or placental malformations. The severity of the prognosis is naturally dependent on the severity of the abnormality.

• The more severe the polyhydramnios, the more likely the chance of finding an underlying cause¹⁰ .