Polyhydramnios And Fetal Anomalies

Introduction

Polyhydramnios is defined as an excess of amniotic fluid surrounding the baby during pregnancy and this is associated with increased foetal anomalies and maternal morbidity.¹ Polyhydramnios is caused by a flaw in the normal amniotic fluid membrane equilibrium whereby there is a disruption in the amniotic fluid being produced and being absorbed.¹

Foetal anomalies can be explained as birth defects which are defined as the abnormal growth in the way a baby develops during pregnancy.² Foetal anomalies are wide-ranging with some defects affecting the physical appearance of the baby, and others the way they think, move, or function.²

It is important to understand the relationship between polyhydramnios and foetal anomalies. Even though polyhydramnios is associated with an increased risk of developing foetal anomalies, in most pregnancies with polyhydramnios no significant problems will develop and the parent will give birth to a healthy baby.³ However, additional health check-ups will be recommended during the pregnancy to make sure that the pregnant parents and baby are healthy.

Causes of Polyhydramnios

Polyhydramnios may be caused by many factors. The characteristic excess amniotic fluid is due to a disruption to the equilibrium of producing and reabsorbing amniotic fluid.⁴ Under normal conditions, the amniotic membrane is produced by foetal urination and foetal lung liquid and reabsorbed by foetal swallowing and intramembranous and intravascular absorption.⁴ The disruption in the amniotic fluid equilibrium could be due to impaired foetal swallowing or an increase in foetal urination, which then leads to polyhydramnios.⁴

Maternal factors

Maternal factors can contribute to the development of polyhydramnios. Diabetes mellitus (type 1 or type 2) and gestational diabetes can lead to polyhydramnios.⁴ High blood sugar in the pregnant parent can cause the foetus to produce more urine in response to the osmotic effect which draws water out of the body.¹ This leads to polyhydramnios as increases in foetal urine lead to an increase in amniotic fluid. Anaemia in the pregnant parent can also lead to polyhydramnios as it can increase foetal urine output.¹

Fetal factors 

Foetal factors can also lead to the development of polyhydramnios. These factors include the structural, functional or developmental abnormalities that can affect how the foetus regulates the amniotic fluid equilibrium. 

1. Gastrointestinal anomalies, where there is an impairment in the swallowing of amniotic fluid, can cause polyhydramnios.⁴ If the foetus is unable to swallow the amniotic membrane, there is a disruption to the normal amniotic fluid equilibrium, causing excess levels of amniotic membrane. Oesophageal atresia and duodenal atresia are examples of conditions that may cause gastrointestinal anomalies⁴
2. Neurological abnormalities are also factors that can lead to polyhydramnios due to an impaired swallowing of the amniotic fluid. Examples include trisomy
3. Renal anomalies are conditions where there is an excess in foetal urination. Bartter syndrome is a rare condition where there is an impairment in sodium absorption causing an abnormally large amount of urine to be produced.¹ This causes an imbalance between the production and absorption of amniotic fluid, causing an excess of amniotic fluid which leads to polyhydramnios

Placental factors

Placental factors can be described as conditions or abnormalities related to the placenta that can cause polyhydramnios. 

1. Chorioangioma is a benign tumor of the placenta and is relatively harmless. Chorioangioma can lead to polyhydramnios as there is an increased blood flow to the tumour, and the increased vascularisation leads to more amniotic fluid being produced⁵
2. Twin-to-twin transfusion syndrome is a condition where the blood flow is unequal between twins that share a placenta. Polyhydramnios occurs due to an increase in urine production and an increase in blood volume (as the blood is shared unequally).¹ Polyhydramnios affect 8-10% of monochorionic twin pregnancies¹

Clinical presentations of polyhydramnios

Maternal symptoms

Some individuals don’t experience many symptoms as polyhydramnios is considered a mild conditions.⁶ Polyhydramnios symptoms develop gradually over time and pregnant parents can experience the following symptoms:^3,6

• Shortness of breath
• Constipation
• Swollen feet and ankles
• Heartburn
• Cramps or tightening in your stomach
• Urinating more often

Foetal symptoms

The clinical presentations of polyhydramnios can include:

• Abnormal foetal positions which make detecting the foetus more difficult¹
• An increase in uterine size due to the excess amniotic fluid
• More foetal movement than usual

Diagnosis and evaluation

There are many techniques available that are used as a diagnostic tool for assessing amniotic fluid levels.

Ultrasound examination

Ultrasound examination is used to measure and evaluate amniotic fluid volumes in the pregnant parent’s uterus.¹ There are two main techniques used to measure this:⁴

1. Single deepest pocket (SDP)
2. Amniotic fluid index (AFI)

SDP

The uterus is split into four quadrants and the volume of amniotic fluid is measured vertically in the deepest amniotic fluid quadrant.⁴ If the volume is more than 8 cm, it means that polyhydramnios is detected. SDP is a simple method and is the most commonly used in clinical practice. This method can also differentiate polyhydramnios into classifications:⁴

1. Mild polyhydramnios when the amniotic fluid volume is 8-11 cm
2. Moderate polyhydramnios when amniotic fluid volume is between 12- 15cm
3. Severe polyhydramnios when the amniotic fluid volume is more than 16 cm

AFI

This method measures the deepest amniotic pocket vertically in each quadrant and the values are added together.⁴ The ultrasound transducer is placed perpendicularly to the pregnant parent’s abdomen.¹ The amniotic fluid measured has to be more than 0.5 cm in width and does not contain the umbilical cord or any foetal limbs.¹ Polyhydramnios is diagnosed when the measured fluid value is more than 25 cm.¹

1. Mild polyhydramnios is when the value is 25 to 30 cm
2. Moderate when the value is 30.1 to 35 cm
3. Severe when the value is more than 35 cm

Once diagnosed, evaluation is required to understand what the underlying cause is.¹ The foetus will need to be examined carefully during screening to assess for any foetal abnormalities. 

Foetal anomalies associated with polyhydramnios

As discussed above, polyhydramnios is associated with several foetal abnormalities caused by two major factors:⁷

1. Reduced foetal swallowing
2. Increase in foetal urination

Reduced foetal swallowing⁷:

• Oesophageal atresia: is a birth defect where the foetuses' esophagus tube is not fully developed. Due to an inability to swallow, there will be an excess of amniotic fluid leading to polyhydramnios being formed
• Duodenal atresia is another birth anomaly where the intestine is not connected to the stomach (an intestinal obstruction). As the foetus is unable to swallow, there will be an excess of fluid causing polyhydramnios to be observed
• Neural tube defects are birth defects where the neural tube does not fully close during development. Examples include anencephaly and Dandy-walker malformation⁷. This can lead to several disruptions in the amniotic fluid equilibrium as it can impair the swallowing of amniotic membrane or disrupt urine production

Increase in foetal urination:⁷

• Maternal diabetes, especially high blood sugar levels in the pregnant parent, can cause the foetus to produce more urine in response to the osmotic effect which draws water out of the body¹
• Hyperdynamic foetal circulation⁷ due to foetal anaemia and examples include red-blood cell isoimmunisation (when the pregnant parent and baby’s blood proteins are incompatible with each other, which causes an immune response to target and target the baby’s blood cells)
• Placental tumours like chorioangioma can lead to polyhydramnios as there is an increased blood flow to the tumour and the increased vascularisation leads to more amniotic fluid being produced⁵

Management and treatment

Treatment is not required for polyhydramnios if the symptoms are mild and if it is towards the end of pregnancy.⁶ Therefore management and treatment will be dependent on the severity of the polyhydramnios based on the diagnosis and what the underlying causes are.¹

Maternal management

Maternal management will include providing relief for the symptoms they are experiencing⁶, for example providing medicine for heartburn or for constipation. 

Antenatal monitoring is a significant focus, especially if the polyhydramnios is moderate or severe¹. This will include regular ultrasound scans to measure the growth of the foetus and its health to determine any health conditions that could affect the pregnant parent or the baby.³ It is also necessary to plan for delivery and pre-birth care. 

Management of underlying conditions is important to treat polyhydramnios. If the pregnant parent has diabetes, then a change in diet or medicine is prescribed to improve the amniotic fluid equilibrium.³

Foetal management

Just like with the pregnant parent, regular surveillance is required to monitor the health of the baby via ultrasound scans. Extra care will be required if polyhydramnios is moderate or severe as neonatal support will be needed, especially if there are any foetal anomalies associated with the birth.¹

Multidisciplinary approaches involving specialists can be incorporated for foetal management, especially if the baby requires neonatal intensive care.¹ This will be required to provide the best care for the baby and also for the pregnant parent. 

Prognosis and outcome

The prognosis is dependent on the severity of polyhydramnios. Mild polyhydramnios prognosis is very good but the prognosis decreases for both the pregnant parent and the foetus with severity of polyhydramnios.¹ In most cases, polyhydramnios is mild and little to no intervention is required, however, pregnancy complications may occur as too much amniotic fluid places pressure into nearby organs.⁵

This can cause the following:⁶

• Early labour and premature birth (before 37 weeks)
• Burst placenta
• Postpartum haemorrhage is a risk that increases with the severity of polyhydramnios
• Collapse of the umbilical cord
• The foetus can become breech
• Stillbirth is unfortunately a risk that increases with the severity of polyhydramnios

Summary

Polyhydramnios is caused by a disruption to the normal amniotic fluid membrane equilibrium where there is a disruption in the amniotic fluid being produced and being absorbed.¹ This is associated with increased foetal anomalies that affect foetal swallowing and urine production. It is important to detect polyhydramnios early on in the pregnancy and to continue monitoring the pregnancy to ensure the best health is provided for the pregnant parent and foetus. Being diagnosed with polyhydramnios is not usually a a cause for serious concern as most polyhydramnios cases are mild and will not need any intervention, however, it will mean an increase in regular antenatal health checkups.