Amniotic fluid: anatomy and functions

Amniotic fluid, or amniotic fluid, is the environment in which the fetus develops within the uterine cavity. It is more than just "water around the baby," but a dynamic system involved in growth, movement, maturation of the lungs and digestive tract, and protection of the umbilical cord from compression. [1]

The composition and volume of fluid changes throughout pregnancy. Early in pregnancy, the membranes and placenta contribute, while later, fetal processes, primarily urine excretion and fluid ingestion, begin to play a leading role. [2]

Amniotic fluid constantly circulates: the fetus swallows it and makes "breathing" movements, after which the fluid returns to the cavity. Therefore, assessing volume is not a "one-and-done" analysis, but rather a measurement of the current balance of formation and utilization. [3]

In practice, amniotic fluid is considered one of the most important indirect indicators of fetal well-being and placental function. A persistent decrease or increase in volume may accompany placental insufficiency, gestational diabetes, congenital anomalies, infections, and rupture of membranes. [4]

Table 1. Approximate guidelines for the volume of amniotic fluid by gestational age

Pregnancy period	Typical volume description	Practical meaning
34 weeks	On average, about 800 ml	Often the maximum volume values
40 weeks	On average, about 600 ml	Physiological decline at full term
Any term	Dynamics and ultrasound measurements are more important than “liters”	Decisions are made based on ultrasound data

[5]

How amniotic fluid is formed and renewed

During the second trimester and especially during the third trimester, the main source of amniotic fluid is fetal urine. If blood flow through the placenta decreases, fetal renal perfusion and urine output decrease, and fluid volume may decrease. [6]

The main route of fluid "exhaustion" is through fetal ingestion and subsequent absorption in the gastrointestinal tract. Therefore, conditions that interfere with swallowing or the passage of contents, such as certain congenital gastrointestinal defects, can lead to polyhydramnios. [7]

There are also additional pathways of exchange through the membranes and the placenta, so the volume of fluid depends not only on “urine and swallowing”, but also on the properties of the membranes, placental blood flow, inflammation and the integrity of the fetal membranes. [8]

It's important to understand that "subjectively little" or "subjectively much" abdominal sensations do not equate to a diagnosis. Some pregnant women with polyhydramnios experience few complaints, while others, despite normal volume, may experience a pronounced feeling of distension due to the position of the fetus, uterine tone, or body type. [9]

Table 2. Balance of formation and utilization of amniotic fluid

Mechanism	Where does the liquid "move"?	What breaks down more often in pathology?
Fetal urine formation	Into the amniotic cavity	Placental insufficiency, some renal anomalies
Fetal ingestion	From the amniotic cavity to the gastrointestinal tract	Gastrointestinal tract defects, neurological disorders
Exchange through membranes and placenta	Between the cavity and the maternal blood flow	Inflammation, dysfunction of the placenta
Loss due to rupture of membranes	Out through the vagina	Leakage and rupture of membranes

[10]

How doctors measure the amount and what the numbers mean

Direct measurement of "liters" is impossible, so ultrasound examination (US) with quantitative methods is used. Two approaches are most common: the amniotic fluid index (AFI) and the largest vertical pocket, also known as the single deepest pocket (SDP). [11]

For oligohydramnios, the following thresholds are often used: AFI 5 cm or less or SDP less than 2 cm. For polyhydramnios, SDP 8 cm or more is used, and when calculating AFI, they focus on a threshold of about 24 cm or higher in the recommendations for polyhydramnios. [12]

The assessment of amniotic fluid is part of the fetal biophysical profile, where one of the criteria for well-being is the presence of a fluid pocket of at least 2 cm. Therefore, “borderline” values usually require re-assessment, comparison with fetal growth, Doppler ultrasound and heart rate, rather than an automatic decision. [13]

Measurements are affected by technique, sensor position, the pregnant woman's position, and even hydration. In some situations, repeating the assessment after fluid intake is recommended, unless contraindicated, to reduce the risk of overdiagnosis of oligohydramnios due to temporary factors. [14]

Table 3. Ultrasound criteria and typical interpretation

Indicator	Norm	Low water	Polyhydramnios
SDP	2-8 cm	less than 2 cm	8 cm and more
AFI	more than 5 cm and less than 24 cm	5 cm and less	24 cm and more, in some sources 25 cm

[15]

Low water levels: causes, risks, and what is usually done

Oligohydramnios is a decrease in the volume of amniotic fluid relative to gestational age, most often diagnosed by ultrasound. It is not an "independent disease," but usually reflects a problem that must be identified: decreased placental blood flow, rupture of membranes, decreased fetal urine output, or congenital anomalies. [16]

One of the common causes in late pregnancy is placental insufficiency and fetal growth restriction. In this situation, fluid loss is assessed along with Doppler ultrasound, biophysical profile, fetal movements, and cardiac monitoring data, and the treatment strategy depends on the severity and stage of pregnancy. [17]

The risks of oligohydramnios depend on the stage and cause. Prolonged, severe fluid deficiency in early pregnancy can lead to impaired limb and lung development; in later pregnancy, intolerance to labor, fetal distress, and a higher risk of operative delivery are more common. [18]

Treatment typically involves identifying the underlying cause, dynamic observation, and monitoring of fetal well-being. Guidelines recommend 1-2 weekly follow-up visits to assess fluid levels and fetal well-being, as well as more frequent growth monitoring if growth restriction is suspected. The decision regarding the timing of delivery is made on an individual basis, particularly if oligohydramnios is isolated and without other complications. [19]

Table 4. Common causes of oligohydramnios and what is usually checked

Possible cause	What might give you an idea?	What is usually assessed
Rupture of membranes	Leakage, wet underwear, decreased fluid on ultrasound	Speculum examination, leakage tests, temperature, signs of infection
Placental insufficiency	Small fetal size, decreased fetal movements	Ultrasound of growth, Doppler ultrasound, biophysical profile
Anomalies of the fetal urinary system	Suspicion based on anatomical ultrasound	Expert ultrasound, discussion of genetic diagnostics as indicated
Medicinal factors and maternal diseases	Hypertension, diabetes, taking certain medications	Assessment of the mother's condition, correction of risk factors

[20]

Polyhydramnios: from "idiopathic" to causes requiring investigation

Polyhydramnios is an increase in amniotic fluid volume, quantitatively confirmed by ultrasound. Guidelines for polyhydramnios in singleton pregnancies use thresholds of SDP 8 cm or greater or AFI 24 cm or greater. [21]

Some cases are idiopathic, meaning no known cause exists, especially in mild cases. However, the detection of polyhydramnios usually prompts a targeted investigation: assessing carbohydrate metabolism, performing a detailed anatomical ultrasound, and, if indicated, considering infections and immunological causes. [22]

Clinical risks may include preterm labor, fetal malposition, cord prolapse when the waters break, and postpartum hemorrhage due to uterine overdistension. The higher the degree of polyhydramnios and the earlier it is detected, the more careful planning is usually required for the management and location of delivery. [23]

The approach depends on the severity and cause. For mild idiopathic polyhydramnios, the guidelines emphasize that this alone does not always require enhanced antenatal monitoring, and that spontaneous delivery is usually allowed at full term. Interventions are used in the presence of symptoms, severity, or an identified cause. It is also specifically noted that indomethacin is not used solely for volume reduction. [24]

Table 5. Degrees of polyhydramnios and typical tactics

Degree	SDP Criteria	AFI criterion	What do they usually do?
Light	8-11.9 cm	24-29.9 cm	Search for the cause, observation, birth plan according to general indications
Moderate	12-15.9 cm	30-34.9 cm	More frequent monitoring, assessment of causes, discussion of the risks of childbirth
Heavy	16 cm and more	35 cm and more	Management at a higher level center, amnioreduction is possible if symptoms are severe

[25]

Membranous Leakage and Rupture: Why It's Critically Important Not to Miss

Leakage and rupture of the membranes, or the loss of water before the onset of labor, is one of the key causes of amniotic fluid loss, especially if the fluid is lost gradually. At full term, this condition occurs in approximately 8% of pregnancies, and the primary maternal risk is associated with intrauterine infection, the risk of which increases with the time from rupture to delivery. [26]

In cases of premature rupture of membranes before 37 weeks, the course depends on the gestational age: the earlier the gestational age, the higher the risks associated with prematurity and infection. A document on rupture of membranes indicates that clinically evident intra-amniotic infection occurs in approximately 15-35% of cases of premature rupture, and postpartum infections in approximately 15-25%. [27]

Diagnosis typically begins with a medical history and speculum examination, with an effort to minimize the risk of infection, as well as an assessment of fetal condition and ultrasound fluid volume as a supporting feature. Additional laboratory tests are used selectively, taking into account the risk of false-positive results and the clinical context. [28]

From a practical standpoint, the most important "red flags" are fever, chills, pain, foul-smelling discharge, bleeding, decreased fetal movements, and leakage associated with prematurity. In such situations, urgent hospital evaluation is usually required, as the treatment strategy depends on the gestational age, signs of infection, and the condition of the fetus. [29]

Table 6. When urgent help is needed if problems with waters are suspected

Symptom or situation	Why is it dangerous?	What do they usually do in the clinic?
Constant leakage of clear fluid	Rupture of the membranes is possible	Speculum examination, tests, fetal assessment
Fever, chills, pain in the lower abdomen, unpleasant odor	Risk of intra-amniotic infection	Analysis, monitoring, decision on tactics
Bloody discharge	Risk of placental complications	Urgent assessment, ultrasound, monitoring
A sharp decrease in movements	Possible fetal harm	Cardiotocography, biophysical profile
Premature term and any signs of leakage	High risks to the fetus	Hospitalization according to indications, individual tactics

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