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By “hydramnios” the topic name refers to polyhydramnios.

Amniotic fluid

Introduction

The amniotic fluid is a clear, yellowish fluid which is present in the amnionic sac after day 12. It has multiple functions, i.e., to cushion the foetus, to facilitate the exchange of nutrients and water between the mother and foetus, to maintain temperature, to assist development of lung, GI system, musculoskeletal system, etc. It’s formed from maternal plasma and foetal urine.

The amount of fluid increases during the pregnancy. The foetus drinks and breathes the fluid in, which also affects the volume of amniotic fluid. At term, there is around 400 mL amniotic fluid.

Assessment

Measuring the volume of amniotic fluid is an important part of routine ultrasound examination. There are two parameters with which we can assess the amount of amniotic fluid, the maximal vertical pocket (MVP) and the amniotic fluid index (AFI).

The (single) maximal vertical pocket (MVP) or single deepest pocket (SDP) is the deepest “pocket” of amniotic fluid (free of foetal parts and umbilical cord) which can be found on ultrasound examination of the mother’s abdomen. (Normal range 2 – 8 cm)

The amniotic fluid index (AFI) is a tool to assess the volume of amniotic fluid. The uterus is divided into four quadrants, and the depth of the deepest pocket of amniotic fluid in each quadrant is measured and added together. (Normal range 8 – 18 cm)

Oligohydramnios

Definition

Oligohydramnios is the condition where the amount of amniotic fluid is decreased compared to normal. This may be associated with (caused by) a variety of maternal, placental, and foetal problems.

It is diagnosed when the MVP is < 2 cm or AFI < 5 cm.

If no amniotic fluid is detected, the condition is known as anhydramnios. This is highly associated with foetal mortality.

Etiology

• Foetal causes
  • Genitourinary abnormalities (bilateral renal agenesis, PKD)
  • Urethral obstruction (posterior urethral valves)
  • Post-term pregnancy
  • Premature rupture of membranes
• Maternal causes
  • Uteroplacental insufficiency (preeclampsia, hypertension, etc.)
  • ACE inhibitors
• Placental causes
  • Twin-twin transfusion syndrome

Pathomechanism

Foetal urine accounts for most of the amniotic fluid, especially in the second half of pregnancy. For this reason, foetal problems which decrease foetal output (genitourinary abnormalities) will reduce the amount of amniotic fluid produced.

Complications

Oligohydramnios may lead to certain complications, depending on the half of pregnancy it occurs in:

• First half of pregnancy
  • Potter sequence
  • Miscarriage or stillbirth
• Second half of pregnancy
  • Intrauterine growth restriction
  • Preterm birth
  • Labour complications
  • Increased risk of perinatal morbidity and mortality

Potter sequence is a group of complications which may occur in case of early pregnancy oligohydramnios. Features include distinct facial features, low-set ears, pulmonary hypoplasia, and limb anomalies. Potter sequence is usually associated with certain causes of oligohydramnios, especially those which decrease foetal renal output (bilateral renal agenesis, PKD, posterior urethral valves).

Management

There is no effective long-term treatment of oligohydramnios. In most cases, regular follow-up and monitoring (with NST) are performed. If the pregnancy is close to term, delivery is often induced.

If delivery is not an option but the condition is too severe to be left untreated, amnio-infusion may be performed. During this procedure, fluid is instilled into the amnionic cavity through amniocentesis. However, this is only temporary.

Polyhydramnios

Definition

Polyhydramnios is the condition where the amount of amniotic fluid is increased compared to normal. Like oligohydramnios, this may be associated with (caused by) a variety of maternal, placental, and foetal problems.

It is diagnosed when the MVP is > 8 cm or AFI > 24 cm.

Etiology

• Idiopathic (60% of cases)
• Foetal causes
  • Isoimmunisation
  • Chromosomal abnormalities
  • TORCH infection
  • Multiple gestation
  • Foetal anaemia
  • Gastrointestinal abnormalities
• Maternal causes
  • Diabetes (gestational or pregestational)
• Placental causes
  • Placental anomaly

Idiopathic polyhydramnios, and polyhydramnios associated with maternal diabetes is usually mild. Polyhydramnios associated with foetal causes is usually severe.

Pathomechanism

Foetal swallowing of amniotic fluid helps regulate the level of amniotic fluid. When foetal swallowing is impaired, due to gastrointestinal abnormalities, chromosomal abnormalities, or CNS abnormalities, amniotic fluid level increases. Increased foetal urination may also contribute to polyhydramnios.

Complications

Polyhydramnios may lead to certain complications:

• Preterm labour
• Macrosomia (excess foetal growth)
• Placental abruption
• Umbilical cord prolapse
• Foetal malposition
• Stillbirth
• Increased risk of perinatal morbidity and mortality

Severe polyhydramnios may also cause abdominal discomfort or shortness of breath for the mother.

Management

A diagnosis of polyhydramnios should prompt investigations to identify an underlying cause, as some of the causes of polyhydramnios can be treated, like bacterial infections, foetal anaemia, and maternal diabetes.

In case of severe polyhydramnios, amnioreduction (amniocentesis to drain excess fluid) may be performed.