Amniotic Fluid Index (AFI)


Article Author:
Megan Lord


Article Editor:
Martha Kole


Editors In Chief:
Anne Kennedy


Managing Editors:
Avais Raja
Orawan Chaigasame
Carrie Smith
Abdul Waheed
Khalid Alsayouri
Kyle Blair
Trevor Nezwek
Radia Jamil
Erin Hughes
Patrick Le
Anoosh Zafar Gondal
Saad Nazir
William Gossman
Hassam Zulfiqar
Navid Mahabadi
Hussain Sajjad
Steve Bhimji
Muhammad Hashmi
John Shell
Matthew Varacallo
Heba Mahdy
Ahmad Malik
Abbey Smiley
Sarosh Vaqar
Mark Pellegrini
James Hughes
Beata Beatty
Daniyal Ameen
Altif Muneeb
Beenish Sohail
Nazia Sadiq
Hajira Basit
Phillip Hynes
Komal Shaheen
Sandeep Sekhon


Updated:
2/1/2019 5:14:12 PM

Introduction

The amniotic fluid index is a standardized way to assess the sufficiency of amniotic fluid quantity in pregnancy. The amniotic fluid index is used in patients who are at least 24 weeks pregnant with a singleton gestation.[1][2][3][4]

Etiology and Epidemiology

Decreased fetal urine output can have a number of causes, which fall into two general categories: fetal urinary tract obstruction and decreased urine production by the fetal kidney. 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 shunting of fetal blood flow away from the kidney to the heart and brain. It is the same mechanism which causes oliguria in critically ill adults.[5][6][7]

Oligohydramnios

When the fetus receives adequate nutrients and oxygen from the placenta, blood is shunted away from the fetal kidney, glomerular filtration rate decreases, and urinary output decreases. Therefore, decreased amniotic fluid volume due to decreased urine production by the fetal kidney is a reflection of chronic hypoperfusion of the fetus.

Oligohydramnios can also occur because the patient's amniotic membrane has ruptured and amniotic fluid is leaking out of the uterus.

Polyhydramnios

The normal fetus is constantly swallowing amniotic fluid and urinating to create more fluid. If the fetus is unable to swallow the typical amounts of amniotic fluid, this can lead to polyhydramnios. This can occur due to gastrointestinal malformations, fetal neurologic problems such as anencephaly, or mechanical obstruction of the esophagus by other intrathoracic processes.

Increased amniotic fluid production occurs as a result of fetal polyuria, such as in uncontrolled maternal diabetes with persistently elevated maternal blood sugars. In these cases, it may be associated with fetal macrosomia.

Many cases of polyhydramnios are idiopathic, meaning no definite cause is identified.

Pathophysiology

Oligohydramnios

Since amniotic fluid is primarily made up of fetal urine, low amniotic fluid volume, or oligohydramnios, typically indicates either fetal urine output or leakage of amniotic fluid from the uterus, such as when the patient's water breaks.

Polyhydramnios

Polyhydramnios, or increased amniotic fluid volume, also has a number of potential causes, with two primary common mechanisms: decreased fetal swallowing of amniotic fluid, or increased fetal production of amniotic fluid. Polyhydramnios can lead to overdistension of the gravid uterus, especially in cases where the fetus is normal size or large for dates, which increases the patient's risk for preterm contractions and preterm delivery, as well as 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.

Specimen Requirements and Procedure

The uterus should be divided into four quadrants to assess amniotic fluid index. Each quadrant should be examined systematically. The ultrasound transducer should be held perpendicular to the patient's spine, not perpendicular to the patient's skin as is performed in most other ultrasound, and should be maintained in an axial plane (notch to the patient's right). This ensures that each pocket of fluid is being measured in the same plane. The deepest vertical pocket of fluid in each quadrant should be identified and measured, and these four measurements should be added together to calculate the total amniotic fluid index. Calipers should be oriented vertically. Color Doppler is typically placed over the pocket of fluid to ensure that the pocket does not contain any segments of the umbilical cord, which are not always well seen in B-mode (standard 2D greyscale) imaging. The calipers may not cross over any segments of the umbilical cord or any fetal parts.

Alternative Measurements of Amniotic Fluid Volume

In pregnancies less than 24 weeks, or with multiple gestations, a single deepest pocket is used. The technique used to measure a single deepest pocket (also referred to as a maximum vertical pocket) is identical to the measurement of amniotic fluid amounts in the four quadrants used to determine an amniotic fluid index. The entire uterus should be examined, and the single deepest vertical pocket of fluid should be identified and measured. A normal single deepest pocket is 2 cm to 8 cm (less than 2 cm is oligohydramnios, greater than 8 cm is polyhydramnios).[8][9]

Interfering Factors

Recall that, in multiple gestations, a single deepest pocket is measured rather than an amniotic fluid index. In twins which share a placenta but which have separate amniotic sacs (monochorionic, diamniotic twins), a sub-type of identical twins, a condition called twin to twin transfusion syndrome (TTTS) may be seen. In this condition, placental blood flow is shunted disproportionately to one twin. The recipient twin becomes overloaded with fluid and attempts to compensate by increasing urine production, leading to increased amniotic fluid volume or polyhydramnios. The donor twin receives inadequate perfusion and shunts blood flow away from the kidneys, leading to decreased amniotic fluid volume or oligohydramnios. This so-called "poly-oly syndrome" is the earliest stage of twin to twin transfusion syndrome. The condition can progress from this point, eventually leading to severe fluid overload and heart failure of the recipient twin, and growth restriction of the donor twin. The death of one or both twins may occur.

Results, Reporting, Critical Findings

Biophysical Profile

Amniotic fluid volume is also part of the fetal biophysical profile, a special type of ultrasound used to assess fetal well-being. The biophysical profile has four sonographic components, each of which must be seen within 30 minutes of starting the ultrasound:

  • Fetal breathing (continuous movement of the fetal diaphragm for at least 30 seconds)

  • Fetal movement (at least three discrete movements of the fetal body or limbs)

  • Fetal tone (at least one active extension of a fetal limb with return to flexion, or opening and closing of the fetal hand)

  • Amniotic fluid volume (single deepest pocket of at least 2 cm)

If available, a fetal non-stress test (fetal heart rate tracing) is also performed with the biophysical profile, for a total of five components.

A healthy, term fetus that is not under physiologic stress would be expected to demonstrate all four of these behaviors on ultrasound. Fetuses which are preterm may not display all of these behaviors, so management of pregnancies in which the fetus does not demonstrate all four of the behaviors described depends on the gestational age, and on which factors are abnormal.

Clinical Significance

A normal amniotic fluid index is 5 cm to 25 cm. Less than 5 cm is considered oligohydramnios, and greater than 2cm is considered polyhydramnios.

Enhancing Healthcare Team Outcomes

Healthcare workers including the nurse practitioner who follow pregnant patients must be aware of the amniotic fluid index. It is an indicator of fetal well being and part of the biophysical profile. When there is suspicion of a problem during pregnancy, the patient should be referred to an obstetrician who may order an ultrasound to determine the AFI. Persistently low levels of AFI during pregnancy may be associated with a birth defect in the fetus. (Level II)


Interested in Participating?

We are looking for contributors to author, edit, and peer review our vast library of review articles and multiple choice questions. In as little as 2-3 hours you can make a significant contribution to your specialty. In return for a small amount of your time, you will receive free access to all content and you will be published as an author or editor in eBooks, apps, online CME/CE activities, and an online Learning Management System for students, teachers, and program directors that allows access to review materials in over 500 specialties.

Improve Content - Become an Author or Editor

This is an academic project designed to provide inexpensive peer-reviewed Apps, eBooks, and very soon an online CME/CE system to help students identify weaknesses and improve knowledge. We would like you to consider being an author or editor. Please click here to learn more. Thank you for you for your interest, the StatPearls Publishing Editorial Team.

Amniotic Fluid Index (AFI) - Questions

Take a quiz of the questions on this article.

Take Quiz
How is the amniotic fluid index (AFI) determined?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up
Which of the following findings would be the most strongly suggestive of oligohydramnios at 33 weeks' gestation?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up
A female who is 32 weeks pregnant presents for a routine ultrasound. The clinician determines the fetus is at risk for a gastrointestinal obstruction anomaly. Which of the following is the most likely amniotic fluid index (AFI)?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up
A 28-year-old primigravid woman at 33 weeks’ gestation is here for her routine prenatal appointment. Her pregnancy has been uneventful thus far. She is taking prenatal vitamins and has felt normal fetal movement. She denies any leakage of fluid or contractions. On exam, she is 5 ft 5 in., 146 pounds, temperature 98.7ºF, BP 121/80, heart rate 67 bpm, and respiration rate 18. Fundal height was measured at 36 cm, and fetal heart tones on Doppler ultrasound were measured at 140-155. What is the next best step to assess the health of the fetus?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up
A 24-year-old primigravid patient presents to labor and delivery triage at 32 weeks' gestation complaining of a severe headache. Her pregnancy has been uncomplicated thus far. She takes no medications. Blood pressure is found to be 182/110 mmHg, and her heart rate is 92/minute. Laboratory evaluation reveals a serum protein-creatinine ratio of 0.42, platelets of 92000/microL, hemoglobin of 12.4 g/dL, creatinine of 1.3 mg/dL, AST of 24U/L, and ALT 14 U/L. Fetal heart rate tracing is category I. Which of the following is most likely to be found on ultrasonic evaluation?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up

Amniotic Fluid Index (AFI) - References

References

Madendag Y,Madendag IC,Sahin E,Aydin E,Sahin ME,Acmaz G, How Well Do the Popular Ultrasonic Techniques Estimate Amniotic Fluid Volume and Diagnose Oligohydramnios, in Fact? Ultrasound quarterly. 2018 Dec 28;     [PubMed]
Cheung CY,Roberts VHJ,Frias AE,Brace RA, Effects of maternal western-style diet on amniotic fluid volume and amnion VEGF profiles in a nonhuman primate model. Physiological reports. 2018 Oct;     [PubMed]
Zhang Y,Yu Y,Chen L,Zhao W,Chu K,Han X, Risk Factors of Intra-Amniotic Infection Related to Induction with Single-Balloon Catheter: A Case-Control Study. Gynecologic and obstetric investigation. 2018 Oct 17;     [PubMed]
Berezowsky A,Ashwal E,Hiersch L,Yogev Y,Aviram A, Transient Isolated Polyhydramnios and Perinatal Outcomes. Ultraschall in der Medizin (Stuttgart, Germany : 1980). 2018 Sep 25;     [PubMed]
Krispin E,Berezowsky A,Chen R,Meizner I,Wiznitzer A,Hadar E,Bardin R, Updating the amniotic fluid index nomograms according to perinatal outcome. The journal of maternal-fetal     [PubMed]
Kehl S, [Single Deepest Vertical Pocket or Amniotic Fluid Index as Evaluation Test for Predicting Adverse Pregnancy Outcome (SAFE trial): A Multicenter, Open-Label, Randomized Controlled Trial]. Zeitschrift fur Geburtshilfe und Neonatologie. 2018 Feb;     [PubMed]
Lee YJ,Kim SC,Joo JK,Lee DH,Kim KH,Lee KS, Amniotic fluid index, single deepest pocket and transvaginal cervical length: Parameter of predictive delivery latency in preterm premature rupture of membranes. Taiwanese journal of obstetrics     [PubMed]
Mousavi AS,Hashemi N,Kashanian M,Sheikhansari N,Bordbar A,Parashi S, Comparison between maternal and neonatal outcome of PPROM in the cases of amniotic fluid index (AFI) of more and less than 5 cm. Journal of obstetrics and gynaecology : the journal of the Institute of Obstetrics and Gynaecology. 2018 Jul;     [PubMed]
Blitz MJ,Rochelson B,Stork LB,Augustine S,Greenberg M,Sison CP,Vohra N, Effect of Maternal Body Mass Index and Amniotic Fluid Index on the Accuracy of Sonographic Estimation of Fetal Weight in Late Gestation. American journal of perinatology. 2018 Nov;     [PubMed]

Disclaimer

The intent of StatPearls is to provide practice questions and explanations to assist you in identifying and resolving knowledge deficits. These questions and explanations are not intended to be a source of the knowledge base of all of medicine, nor is it intended to be a board or certification review of Radiology-Ultrasound. The authors or editors do not warrant the information is complete or accurate. The reader is encouraged to verify each answer and explanation in several references. All drug indications and dosages should be verified before administration.

StatPearls offers the most comprehensive database of free multiple-choice questions with explanations and short review chapters ever developed. This system helps physicians, medical students, dentists, nurses, pharmacists, and allied health professionals identify education deficits and learn new concepts. StatPearls is not a board or certification review system for Radiology-Ultrasound, it is a learning system that you can use to help improve your knowledge base of medicine for life-long learning. StatPearls will help you identify your weaknesses so that when you are ready to study for a board or certification exam in Radiology-Ultrasound, you will already be prepared.

Our content is updated continuously through a multi-step peer review process that will help you be prepared and review for a thorough knowledge of Radiology-Ultrasound. When it is time for the Radiology-Ultrasound board and certification exam, you will already be ready. Besides online study quizzes, we also publish our peer-reviewed content in eBooks and mobile Apps. We also offer inexpensive CME/CE, so our content can be used to attain education credits while you study Radiology-Ultrasound.