Tools
Sonography Evaluation of Amniotic Fluid
Introduction
Amniotic fluid is critical during pregnancy to provide a protective and nourishing environment for the developing fetus. It maintains a sterile space, regulates temperature, and cushions against external shocks. Amniotic fluid facilitates fetal movement, essential for musculoskeletal development, and supports the growth of organs, primarily the lungs.[1][2][1] The amount of amniotic fluid gradually increases until approximately 34 weeks gestation, after which it slightly decreases until 40 weeks and then declines more sharply after 42 weeks.[3]
In the early stages of pregnancy, the composition of amniotic fluid resembles a complex dialysate derived from maternal serum. As the fetus grows, changes occur in the fluid's composition; notably, sodium concentration and osmolality decrease while urea, creatinine, and uric acid levels increase.[4][5] Amniotic fluid also contains various steroid and protein hormones.[6][7] Initially, it has little to no particulate matter, but by 16 weeks gestation, a significant number of cells shed from the amnion, skin, and tracheobronchial tree are present. These cells are crucial for antenatal diagnosis and serve as a source of DNA for karyotype analysis following amniocentesis.[8] Typically, fetuses do not defecate during pregnancy; however, if under severe stress, they may pass meconium. This material contains bile pigments that can stain the amniotic fluid green, indicating fetal stress.[9] The regulation of amniotic fluid volume (AFV) involves 3 primary mechanisms: placental control of water and solute transfer, fetal contributions through urine production and swallowing, and maternal factors affecting fluid balance.
Before 16 weeks of gestation, amniotic fluid is maintained primarily through intramembranous flow, transitioning to fetal urine production as the kidneys mature. The evaluation of amniotic fluid can be an indicator of fetal well-being.[10] Maternal and placental factors, including serum osmolality, blood pressure, and placental vascularity, also influence AFV by modulating intramembranous flow. Conditions like maternal dehydration or altered placental function can disrupt this balance, leading to changes in amniotic fluid levels.
Clinical assessment of AF is essential to detect oligohydramnios or polyhydramnios, which can signal underlying fetal or maternal conditions. Ultrasound offers a rapid, noninvasive way to evaluate amniotic fluid. Some studies have noted that sonographic assessment of amniotic fluid may not be accurate compared to the more direct dye dilution method, as ultrasound measurements are less accurate in evaluating abnormal amniotic fluid volumes than normal volumes.[11][12] However, ultrasound offers a safe real-time option with comparable clinical outcomes.[13][14] Sonographic assessment can be qualitative, visually assessing the volume ratio of amniotic fluid to the fetus and placenta, or semiquantitative. The 2 main methods for semiquantitative evaluation are the deepest vertical pocket (DVP) and the amniotic fluid index (AFI). Current guidelines recommend using DVP to diagnose oligohydramnios and AFI for polyhydramnios in singleton pregnancies, as studies indicate that these methods reduce the risk of overdiagnosis. For twin pregnancies, DVP is measured separately for each sac.[12] Therefore, a thorough understanding of amniotic fluid production, regulation, and assessment is crucial for optimizing maternal-fetal care, as abnormalities in AFV can have significant implications for perinatal outcomes.
Anatomy and Physiology
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Anatomy and Physiology
Amniotic fluid Physiology
At the beginning of pregnancy, amniotic fluid is primarily a byproduct of maternal serum and resembles plasma due to hydrostatic and osmotic pressures. After 8 weeks of gestation, the fetal kidneys begin to function and produce urine; however, this does not become a key component of amniotic fluid until closer to 23 to 25 weeks, when fetal keratinization is complete.[1] By the time keratinization is complete, fetal urinary production accounts for the most significant amount of amitotic fluid, with up to 1500 mL produced daily.[15] In the last 20 weeks, fetal respiratory and gastric secretions also contribute up to 350 mL per day. Amniotic fluid is recycled primarily by fetal swallowing.[1]
Additional recycling pathways include the intramembranous pathway of the placenta, umbilical cord, and fetal skin between the amniotic fluid and fetal blood (400 mL per day).[15] Amniotic fluid increases at a linear rate initially in the first half of pregnancy and continues to increase until the 36th or 38th week of pregnancy with an average maximal volume of 800 mL.[10][15] Next, amniotic fluid can decrease by 8% per week, down to as low as 400 mL in gestation, near week 42.[10]
Under normal conditions, maternal serum remains near but slightly lower in osmolality to fetal serum, favoring flow to the fetus. Following fetal keratinization, amniotic fluid remains 10 Osm lower than fetal blood, allowing continuous flow through the placenta to the fetal blood.[10] In maternal dehydration, this balanced gradient between maternal and fetal serum can reverse, favoring flow out of the fetal circulation. Counter-regulation by continued flow into fetal circulation can decrease amniotic fluid volume when this occurs. These osmolarity gradient changes may explain why maternal hydration effectively reduces isolated uncomplicated oligohydramnios and why hyperglycemia often results in polyhydramnios.[15] The fetus can also affect the flow through these pathways due to the excretion of certain hormones, like growth factors and prolactin, which can alter membranes' permeability.
Oligohydramnios
Decreased fetal urine output can arise from several causes, which fall into 2 general categories—fetal urinary tract obstruction and decreased urine production by the fetal kidney.[16] The inability to visualize any amniotic fluid is termed anhydramnios. Urinary tract obstruction can occur anywhere along the fetal urinary tract and can be catastrophic for the fetus. Decreased urine production by the fetal kidney typically reflects inadequate blood flow to the fetal kidney, caused by the shunting of fetal blood flow away from the kidney to the heart and brain. When the fetus receives insufficient nutrients and oxygen from the placenta, blood is redirected away from the fetal kidney, decreasing the glomerular filtration rate and reducing urinary output. Therefore, decreased amniotic fluid volume due to decreased urine production by the fetal kidney is a reflection of chronic hypoperfusion of the fetus.[17][18] This mechanism mirrors the cause of oliguria in critically ill adults.[19][20]
Another common cause of oligohydramnios is the rupture of the amniotic membrane, which allows amniotic fluid to leak out of the uterus.[21][22] Preterm premature rupture of membranes (PPROM) can occur spontaneously or as a result of medical procedures (eg, amniocentesis). When PPROM occurs after 25 weeks, survival outcomes are more favorable, improving as gestational age advances, as the fetus has more time for normal lung development.[12]
Polyhydramnios
Polyhydramnios, or increased amniotic fluid volume, also has several potential causes, with 2 primary mechanisms—decreased fetal swallowing of amniotic fluid or increased fetal production of amniotic fluid.[23] Polyhydramnios can cause overdistension of the gravid uterus, especially in cases where the fetus is normal size or large for dates. This overdistension increases the patient's risk for preterm contractions, preterm delivery, and premature rupture of membranes, in which the patient's water breaks before the onset of labor. Overdistension of the uterus is also a risk factor for postpartum hemorrhage after delivery.[24][25]
The normal fetus constantly swallows amniotic fluid and urinates to maintain the fluid level. If the fetus is unable to swallow the typical amounts of amniotic fluid, it can lead to polyhydramnios. This condition can occur due to gastrointestinal malformations, fetal neurological issues, eg, anencephaly, or mechanical obstruction of the esophagus by other intrathoracic processes.[23] Increased production of amniotic fluid can occur due to fetal polyuria, often observed in cases of uncontrolled maternal diabetes with persistently elevated maternal blood sugar levels. In these cases, it may be associated with fetal macrosomia. Many cases of polyhydramnios are idiopathic, meaning no definite cause is identified.[26]
Indications
Amniotic fluid assessments are considered integral to the routine anatomic fetal evaluation, with derangements often found incidentally. Several professional society guidelines state that amniotic fluid assessment should be performed at all sonographic fetal evaluations.[27][18][27] Specific high-risk factors in pregnancies have the potential to alter amniotic fluid dynamics and may warrant serial assessments. These high-risk factors include hypertension severe enough to require antihypertensives, pregnancy-induced hypertensive disorders, fetal growth restriction, certain fetal anatomic anomalies, and late-term pregnancies. Additionally, when fundal size on a routine exam does not correlate with the gestational size, the amniotic fluid volume should be assessed.[28] The assessment of amniotic fluid volume can also help rule out rupture of membranes and is included in the general assessment of fetal wellbeing as 1 of the 5 parts of the biophysical profile.
Contraindications
Although diagnostic ultrasound is generally considered safe in pregnancy, ultrasound should only be performed when medically indicated. The concept of as-low-as-reasonably-achievable should be applied to reduce the overall thermal index exposed to the fetus.[29] Doppler should not be used unless necessary due to the high-energy output of this technique.[30]
Equipment
Performing a diagnostics ultrasound in pregnancy requires real-time scanning. For subjective fluid measurement in the first trimester of pregnancy, intracavitary transducers may be used. Most amniotic fluid assessments will occur in the second and third trimester and utilize a transabdominal approach with a curvilinear transducer able to provide greater than 3 MHz.[29]
Technique or Treatment
Amniotic Fluid Volume Assessment Techniques
To assess amniotic fluid using sonography, the patient should be positioned supine, with the transducer held perpendicular to the floor, and aligned coronally to the patient. For the DVP, also referred to as the maximum vertical pocket (MVP), the entire uterus is scanned to identify the amniotic fluid pocket with the greatest vertical length. This pocket, which appears hypoechoic on ultrasound, is measured using vertical sonographic calipers.[31] The measurement process must ensure that the calipers do not cross over fetal body parts or portions of the umbilical cord, as this could overestimate the pocket size. Pockets must be at least 1 cm wide to be considered valid, except for biophysical profiles, where a 2 cm width is required.[32][12][32]
To calculate the AFI, the uterus is divided into 4 quadrants using the umbilicus as the central point. To ensure measurement consistency, the transducer should remain in an axial plane (notch toward the patient’s right). Each quadrant is systematically scanned to identify the largest vertical pocket of fluid, which is then measured with vertical calipers. The 4 measurements are added together to determine the total AFI.[33] Color Doppler is recommended to confirm that the measured pockets are free of umbilical cord segments, which may not be visible on standard B-mode imaging.[12][18][34][12]
A DVP measurement is used for pregnancies <24 weeks or those with multiple gestations.[35] The procedure is similar to that for AFI, but only a single DVP is identified and measured. A normal DVP ranges from 2 to 8 cm, with values <2 cm indicating oligohydramnios and >8 cm indicating polyhydramnios.[36][37]
Amniotic fluid measurements can vary due to intra-observer differences, maternal position, and hydration status. Maternal hydration, especially with simple hypotonic oral solutions, can improve the specificity of oligohydramnios diagnosis.[27][38] Therefore, encouraging the patient to consume several liters of water before the evaluation may enhance accuracy.[12][39][12]
Biophysical Profile
Amniotic fluid volume is an essential component of the fetal biophysical profile, a specialized ultrasound technique used to assess fetal well-being. The biophysical profile has 4 sonographic components, each of which must be observed within 30 minutes of starting the ultrasound:
- Fetal breathing: Continuous movement of the fetal diaphragm for at least 30 seconds)
- Fetal movement: At least 3 discrete movements of the fetal body or limbs)
- Fetal tone: At least 1 active extension of a fetal limb with the return to flexion or opening and closing of the fetal hand
- Amniotic fluid volume: A single deepest pocket of at least 2 cm [40][41]
A fetal nonstress test measuring fetal heart rate tracing is also performed with the biophysical profile for a total of 5 components.[42]
A healthy, term fetus that is not under physiological stress is expected to demonstrate all 4 of these behaviors on ultrasound. Preterm fetuses may not display all of these behaviors. Therefore, the management of pregnancies in which the fetus does not show all 4 behaviors described depends on the gestational age and the specific factors that are abnormal.[43]
Complications
Complications during the sonographic assessment of AFI or DVP are minimal. When performing the ultrasound, patients may experience discomfort with the pressure from the ultrasound probe or lightheadedness from the gravid uterus's compression on the vena cava in the supine position. A rolled towel under the patient’s flank to provide left lateral tilt may improve venous return and improve symptoms.[44]
Clinical Significance
Interpretation of Abnormal Amniotic Fluid Measurements
Normal reference ranges for the AFI vary depending on if the 5 to 95 or 3 to 97 percentiles are applied. For single gestations above 20 weeks, normal fluid is noted as an AFI between 5 to 24 cm, although some references prefer 25 cm as the upper limit. MVP in single-gestation pregnancies is considered normal when measured between 2 and 8 cm. It is important to note that only MVP is used to assess the amniotic fluid volumes in multi-gestation pregnancies. In a diamniotic pregnancy, the MVP is measured in each amniotic sac, allowing the clinician to attribute fluid to each fetus. Although the normal range for twins is closer to 2.2 to 7.5 cm, it is conventionally defined as between 2 to 8 cm.[31][45]
Oligohydramnios is diagnosed when amniotic fluid values fall below normal cut-offs. The list of potential causes is long and includes any condition causing uteroplacental insufficiency, anatomic issues with fetal urine production, and urinary tract obstructions. In severe cases, early pulmonary hypoplasia and limb disfigurement occur, resulting in significant perinatal morbidity and mortality. Even among structurally normal fetuses, oligohydramnios' mortality has been shown to have a 50-fold increase compared to a fetus with normal amniotic fluid measurements.[46] Patients with oligohydramnios have several additional complications, including higher rates of intrauterine growth restriction, fetal distress on heart rate monitoring, meconium-stained amniotic fluid, low Apgar scores, and higher rates of cesarean section.[46][47][48][49]
Management of Abnormal Amniotic Fluid Volumes
As a result of these complications, the American College of Obstetrics and Gynecology (ACOG) recommends induction of pregnancy for isolated oligohydramnios between 36.0 and 37.6 weeks.[50] Measurements must be obtained accurately, as these can lead to unnecessary intervention. Multiple studies have compared the use of the AFI with MVP for the diagnosis of oligohydramnios. Results indicate that AFI, compared to MVP, tends to overestimate clinically significant oligohydramnios and leads to overdiagnosis without improving maternal and perinatal outcomes.[35][51] Therefore, MVP is the preferred method for diagnosing oligohydramnios.[31]
Polyhydramnios results from conditions in which there is increased production of amniotic fluid or decreased fetal swallowing. Causes of overproduction can include increased osmotic forces (eg, hyperglycemia or hydrops), cardiac dysfunction, infection leading to decreased fetal swallowing (occurs due to gastrointestinal tract obstruction), impaired neurological swallowing pathways, and some cranial facial abnormalities. The majority of cases are mild with no underlying etiology. As the severity of polyhydramnios increases, so does the incidence of fetal structural and genetic anomalies. Polyhydramnios can be graded in severity using the following thresholds for AFI or DVP:
- AFI
- Mild 24 to 29.9 cm
- Moderate 30 to 34.9 cm
- Severe ≥35 cm
- DVP
- Mild 8 to 11 cm
- Moderate 12 to 15 cm
- Severe: >16 cm [28]
Some evidence suggests that the MVP method may overestimate polyhydramnios, but more data is needed to confirm this hypothesis.[38] Polyhydramnios has been shown to increase perinatal mortality to a lesser extent than oligohydramnios, with the rate dependent on the underlying cause. Perinatal morbidity associated with polyhydramnios includes an increase in preterm labor, macrosomia, nonvertex positions, nonreassuring fetal heart tracings, and cesarean delivery.[48] Polyhydramnios can cause significant discomfort to the mother as severe cases can impair adequate maternal respiration. In these cases, therapeutic amnioreduction may be considered. For most causes of mild polyhydramnios, ACOG currently recommends delivery between 39 and 39.6 weeks.[50]
Abnormal Amniotic Fluid Management in Multiple Gestations
Twin-twin transfusion syndrome is a special case that warrants mentioning in the discussion of amniotic fluid derangement due to its simultaneous presentation of both polyhydramnios and oligohydramnios during the same pregnancy. This syndrome occurs in 10% to 15% of diamniotic-monochorionic twins due to shared chorionic vessels.[52] The diagnosis requires a single placenta and the findings of oligohydramnios in 1 twin and polyhydramnios in the other. Twin-twin transfusion syndrome is often associated with hydrops of the twin with polyhydramnios and growth restriction of the twin with oligohydramnios. Due to significant associated maternal and fetal mortality and morbidity, current guidelines recommend screening starting at 16 weeks and continuing every 2 weeks until delivery to allow for early identification and treatment to guide delivery timelines.[52]
Enhancing Healthcare Team Outcomes
The assessment and management of amniotic fluid levels, including oligohydramnios and polyhydramnios, require a coordinated interprofessional approach to ensure optimal patient-centered care, safety, and outcomes. These conditions, which can have profound implications during pregnancy, demand heightened vigilance from healthcare team members. Physicians, including obstetricians, maternal-fetal medicine specialists, and pediatric radiologists, play a pivotal role in performing and interpreting detailed anatomic ultrasounds to detect fetal anomalies commonly associated with abnormal amniotic fluid levels. Advanced practitioners, eg, nurse practitioners, must also be proficient in understanding and assessing the amniotic fluid index as part of the biophysical profile to monitor fetal well-being and identify potential complications.
Effective care requires robust interprofessional communication and collaboration. When abnormalities in amniotic fluid are suspected, referrals to obstetricians or maternal-fetal medicine specialists for advanced imaging and evaluation are critical. In cases of severe oligohydramnios or polyhydramnios, care coordination must include neonatal clinicians and personnel skilled in neonatal resuscitation, eg, neonatal intensive care unit (NICU) trained nurses and specialized physicians, to manage potential complications during delivery. Delivery in a tertiary care facility is often necessary due to the higher likelihood of NICU admissions and postnatal anomalies.
The healthcare team’s responsibilities include ensuring patient safety and optimizing outcomes through comprehensive care coordination. This involves timely identification of at-risk pregnancies, appropriate referrals, and interdisciplinary discussions to develop individualized care plans. By fostering a team-based approach that integrates the expertise of physicians, advanced practitioners, nurses, and other health professionals, the healthcare system can effectively address the complexities of amniotic fluid abnormalities. This collaboration enhances team performance and ensures that maternal and neonatal outcomes are prioritized throughout the continuum of care.
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