Amniotic fluid is the protective liquid surrounding a developing fetus within the uterus. It supports fetal growth and development by facilitating lung and digestive system maturation, enabling movement, and cushioning the fetus. When the fluid volume becomes excessively high, the condition is known as polyhydramnios. It is typically diagnosed when the amniotic fluid index (AFI) is greater than 24 cm, or the deepest vertical pocket of fluid measures at least 8 cm. Polyhydramnios occurs in approximately 1% to 2% of pregnancies, most commonly in the second and third trimesters.
Amniotic Fluid Regulation
The volume of amniotic fluid is regulated through a continuous balance of production and absorption. In early pregnancy, it is primarily composed of water from the mother’s body. After 20 weeks, fetal urine, produced by the kidneys, becomes the main contributor to fluid volume.
The primary mechanism for fluid removal is fetal swallowing. A fetus at term can swallow 200 to 450 ml of amniotic fluid daily, absorbing it through its gastrointestinal system. This swallowed fluid is either processed through the fetal kidneys or transferred back to the maternal circulation via the placenta. This interplay ensures the amniotic fluid volume remains stable, supporting fetal well-being.
Maternal Factors
Several maternal health conditions can disrupt the balance of amniotic fluid, leading to polyhydramnios. Maternal diabetes, including pre-existing and gestational diabetes, is one factor. Elevated maternal glucose levels cross the placenta, causing fetal hyperglycemia and increased fetal urine production (osmotic diuresis), which raises fluid volume. Maternal diabetes accounts for an estimated 8% to 25% of polyhydramnios cases.
Rh incompatibility is another maternal condition. This occurs when a blood type mismatch between mother and fetus leads to fetal anemia. Severe fetal anemia can result in high-output cardiac failure and increased fetal urine production. In monochorionic twin pregnancies, twin-to-twin transfusion syndrome (TTTS) can also cause polyhydramnios. In TTTS, unequal blood flow through shared placental vessels causes one twin (the recipient) to receive too much blood. This volume overload leads the recipient twin to produce excessive urine, resulting in polyhydramnios, while the donor twin experiences oligohydramnios.
Fetal Conditions
Fetal anomalies can impact amniotic fluid regulation by impairing the fetus’s ability to swallow or by increasing fetal urine output. Conditions obstructing the fetal gastrointestinal tract prevent normal fluid absorption. Examples include esophageal atresia (esophagus not connecting to stomach) and duodenal atresia (small intestine obstruction). In these cases, the fetus cannot swallow the fluid, leading to its accumulation.
Neurological impairments can also affect the swallowing reflex. Conditions like anencephaly, a severe neural tube defect, can lead to ineffective fetal swallowing. Other neurological or neuromuscular disorders, such as myotonic dystrophy, can similarly impair the fetus’s ability to clear amniotic fluid. Additionally, certain fetal anomalies or conditions, including infections and some chromosomal abnormalities, may increase fetal urine production or elevate fluid volume through other mechanisms.
Idiopathic Cases
Despite thorough medical investigation, a specific cause for increased amniotic fluid cannot always be identified. These instances are categorized as idiopathic polyhydramnios. This classification applies after ruling out other potential factors, such as maternal diabetes and fetal anomalies. Idiopathic cases represent a substantial proportion of polyhydramnios diagnoses, with some studies indicating that 50% to 70% of cases have no discernible cause.
Mild polyhydramnios is particularly likely to be idiopathic, with up to 80% of mild cases having no identified etiology. Although a cause is not found, monitoring of the pregnancy remains important. While the prognosis for mild idiopathic polyhydramnios is generally favorable, a fetal anomaly may only be diagnosed after birth in a small percentage of these cases.