How Can Ultrasound Aid in the Diagnosis of Polyhydramnios?
Polyhydramnios describes an excess of amniotic fluid within the amniotic sac that can occur during the second or third trimesters of pregnancy. The diagnosis of polyhydramnios is made through ultrasound and is determined by comparing the gestational age of the fetus with the amount of fluid visible.
Amniotic fluid is constantly circulated and replaced through fetal swallowing and absorption by tissues. In a typical pregnancy, the volume of amniotic fluid gradually increases throughout the first and second trimesters. Then, early in the third trimester, the fluid volume remains stable and then gradually decreases throughout the late third trimester.
Amniotic fluid surrounds the fetus throughout its development — also critical to fetal lung development — it is essential for protection, temperature regulation and fetal movement.
The Role of Ultrasound in Polyhydramnios
Ultrasound is an accurate and safe diagnostic test to use during pregnancy, primarily because it is noninvasive and radiation free. This imaging modality also allows for a live view of the fetus, placenta and amniotic fluid, ensuring that the most accurate measurements can be made and that multiple follow-up exams can be safely performed.
During an ultrasound exam, the diagnosis of polyhydramnios can be performed in two ways:
1.Visually "eyeballing" the amount of fluid. In the second trimester, the normal amount of amniotic fluid is approximately equal to the fetal size or a 1:1 fluid-to-fetus ratio. If there is enough fluid to accommodate multiple fetal trunk cross-sections — or if the fetus is able to stretch its legs or arms out fully straight — then it indicates polyhydramnios is present.
2.Vertical measurements of fluid pockets. The single deepest pocket (SDP) evaluation is a measurement taken from the anterior to posterior edges of the largest pocket of amniotic fluid. The fluid pocket being measured cannot contain fetal parts or umbilical cord. An SDP greater than 8 cm is a sign of polyhydramnios. This can be combined with the amniotic fluid index (AFI), which is the sum of the four deepest vertical pocket measurements — one done in each quadrant around the fetus. An AFI of greater than 20 cm is another sign of polyhydramnios.
Causes of Polyhydramnios
Polyhydramnios is associated with conditions that interfere with the intake and absorption of amniotic fluid. Anomalies that increase fetal urinary or respiratory fluid production include maternal diabetes, which causes fetal polyuria. Conditions that decrease fetal swallowing include gastrointestinal obstructions, neural tube defects or muscular abnormalities that affect the contractibility of muscles.
A rare association with polyhydramnios is cord prolapse into the endocervical canal at delivery. Polyhydramnios is also associated with premature delivery, premature rupture of membranes and placental abruption, which can increase the risk of fetal morbidity and mortality.
How Ultrasound Supports Polyhydramnios During Pregnancy
Ultrasound is able to detect and document many of the abnormalities that can lead to polyhydramnios. If excess amniotic fluid is ever suspected, a detailed assessment of the fetal gastrointestinal tract, renal system, skeletal and central nervous system may be performed to determine if an anomaly in one of those areas is the cause. Ultrasound is used for repeated follow-up assessments as well as for measurements of the fetus and amniotic fluid to determine if abnormalities are progressing.
Cases of mild idiopathic polyhydramnios typically have an excellent prognosis. However, polyhydramnios treatment and outcome depend on the degree of severity and the cause. Using ultrasound to assess amniotic fluid amounts is integral to the biophysical profile and is often used in conjunction with a nonstress prenatal test to monitor and determine fetal well-being.
