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How To Measure AFI In Twin Pregnancy


Assessment of amniotic fluid index(AFI) is an essential part of the fetal anatomic survey, growth assessment, and antenatal testing in twin pregnancies because of the increased risk of spontaneous abortions, malformations, low birth weight neonates, and preterm deliveries compared to singleton pregnancies.

There is an increased likelihood of complications such as gestational diabetes, high blood pressure/preeclampsia, acute fatty liver, and placental abruption.

In this article, we'll discuss how to measure amniotic fluid index(AFI) in twin pregnancy. But first, let's discuss the definition of AFI, amniotic fluid, and how important it is.

What Is Amniotic Fluid?

A fetus surrounded by amniotic fluid inside the womb
A fetus surrounded by amniotic fluid inside the womb

COPYRIGHT_WI: Published on https://washingtonindependent.com/ebv/how-to-measure-afi-in-twin-pregnancy/ by Karan Emery on 2022-08-23T08:25:55.543Z

Amniotic fluid is the fluid that surrounds a fetus during pregnancy.

Because of fluid transudation through the fetal skin or the maternal decidua, the composition of amniotic fluid in the first trimester of pregnancy is comparable to that of protein-free fetal serum.

At roughly 11 weeks of pregnancy, amniotic fluid is virtually entirely fetal urine, with only a minor amount of fluid produced by the lungs.

Amniotic fluid content appears to be regulated by a balance between fetal urine inflows and outflows from fetal swallowing and intramembranous water flow (from amniotic fluid over the amnion into the fetal-placental vasculature).

Amniotic fluid accumulates during the first and second trimesters of pregnancy, reaching its peak between weeks 33 and 34.

Amniotic Fluid Index Measurement

According to Phelan and Rutherford's theory, the amniotic fluid index is calculated by adding the vertical diameters of the greatest pockets in each of the four quadrants.

The transducer must be oriented in the longitudinal axis of the mother's abdomen and kept perpendicular to the floor throughout scanning.

It employs the mother's umbilicus as its focal point.

Although it loses predictive power at the higher and lower ends of the extreme, the AFI roughly matches the real amniotic fluid volume as measured by dye-dilution procedures for normal fluid volume.

In a study of 791 healthy pregnancies, Moore and Cayle defined the mean and outer boundaries (5th and 95th percentiles, respectively) for the AFI from 16 to 42 weeks of gestation.

AFI at gestational age 37–41 weeks was measured in the 5th–95th percentiles, which were set by Magann et al. as 4.2-14.9 cm, respectively.

During the majority of pregnancy, the mean AFI is between 12 and 14 cm, however it starts to decline after 33 weeks.

There are roughly 20 cm above the 95th percentile and 7 cm below the 5th percentile.

Less than 24 or 25 cm is the absolute maximum for a normal AFI.

Normal Amniotic Fluid Volume In Twins

Baby twins wrapped up in different colored cloth sleeping together
Baby twins wrapped up in different colored cloth sleeping together

Normal amniotic fluid volume must be established before aberrant amniotic fluid volume in twin pregnancies may be evaluated.

The amniotic fluid volume in diamniotic twin pregnancies was the subject of the sole study undertaken to answer this topic.

Researchers found that the quantities of individual amniotic fluid sacs ranged from 155 to 5430 ml, with a mean of 877 ml, which is comparable to that seen in singleton pregnancies.

The volumes of the individual sacs were calculated using the methods pioneered by Charles and Jacoby, who used dyes to measure fluid movement.

This technique is too intrusive and requires the aid of a lab to be practical for monitoring the volume of amniotic fluid on a daily basis.

As a more practical replacement, ultrasound-based amniotic fluid volume estimation emerged.

Multiple ultrasound-based methods have been developed for estimating the amniotic fluid volume in twins, including the amniotic fluid index, the single deepest pocket (SDP), the two diameter pocket (2DP), and the subjective assessment of the amniotic fluid volume.

Estimation Of Amniotic Fluid Amount In Twins Using Ultrasound

A pregnant woman having an ultrasound with the help of a healthworker
A pregnant woman having an ultrasound with the help of a healthworker

The Phelan method is typically used in singleton pregnancies to estimate the amniotic fluid volume by the AFI.

The uterus is an organ, split into four quadrants: upper and lower quadrants by the umbilicus, and right and left halves by the linea nigra.

The greatest vertical pocket of fluid within each quadrant without an accumulation of cord or fetal tiny parts is measured in cm with the ultrasound transducer held perpendicular to the floor.

Clinical evaluation of AFV based on the Leopold maneuver or fundal height measurements has mostly been supplanted by ultrasound.

However, if the uterus is unusually big or too small for the gestational age, AF problems should be suspected.

If the uterus of the mother is large for the gestational age (LGA), or if the fetus is ballotable or difficult to palpate, polyhydramnios may be present.

When the fundal height is small for gestational age (SGA) or the fetus is easily palpable, oligohydramnios may be suspected.

The positioning of the membrane is not taken into consideration while computing the summated AFI.

The summated AFI correctly recognized 94% of twin pairs as having normal amniotic fluid volume, despite the fact that only 52% of twin pairs had normal volumes when dye-dilution procedures were employed to test the accuracy of the summated AFI.

There are various different methods that fall within the category of AFI that have been utilized to estimate the amniotic fluid volume in twins.

Gerson and colleagues measured the largest pocket free of the umbilical cord in centimeters, located the separating membrane between the twins, divided the amniotic fluid sac into upper and lower halves using the fetal diaphragm, and added the measurements to obtain a two-quadrant AFI.

Each embryo and its surrounding amniotic sac were identified by Hill et al.

They then measured the greatest vertical pocket in each of the four quadrants of the sac, using the fetus as the vertical axis in the sac.

These 4 measurements were added together to form the AFI.

The efficacy of either of these approaches has not been evaluated using a dye-determined amniotic fluid volume.

The Single Deepest Pocket (SDP)

In singleton pregnancies, the Chamberlain method is typically employed to calculate the volume of amniotic fluid.

The largest fluid pocket's vertical and transverse dimensions were determined by angling the ultrasound transducer at a right angle to the uterine contour.

The largest pocket of amniotic fluid was identified by measuring its breadth at a right angle to its depth.

With the exception of cases of severe oligohydramnios (vertical pocket of 1 cm), all pockets were less than 1 cm wide.

A depth of less than one centimeter was used to define severe oligohydramnios, 1-2 centimeters was used to define oligohydramnios, and more than two centimeters to eight millimeters was used to define normal.

In a related article that was also published in the same publication, polyhydramnios was discovered as the greatest vertical pocket out of the eight.

With the umbilical cord, minor components of the fetus, or simply if their appearance is transient, many clinicians now measure the largest pocket.

When calculating amniotic fluid volume using the SDP in twins, the separating membrane is identified and the size of the biggest vertical pocket of amniotic fluid volume in each sac is determined in centimeters.

A measurement of less than 2 cm is considered oligohydramnios, 2–8 cm is considered normal, and more than 8 cm is considered hydramnios.

The measurement thresholds are the same as those used in singletons.

In 47 of 48 cases (98%), the SDP accurately recognized fluid volumes ranging from 500 to 2000 ml; however, it misidentified volumes of less than 500 ml in 1 of 35 cases (3%), and volumes of more than 2000 ml in 0 of 7 cases (0%).

2-Diameter Pocket

To calculate the 2-diameter pocket approach in singleton pregnancies, multiply the vertical measurement by the horizontal dimension of the largest discovered pocket of amniotic fluid.

Two-diameter pockets measuring less than 15 centimeters in diameter were considered to be indicative of oligohydramnios, whereas those measuring 15 to 50 centimeters were considered to be indicative of normal fluid volume.

In twin pregnancies, the horizontal measurement is multiplied by the vertical measurement to determine the total size of each sac after separating the membrane and locating the largest pocket of fluid in each sac without fetal tiny parts or the umbilical cord.

Oligohydramnios, normal, and polyhydramnios are all assigned the same numerical values for identical and fraternal twins.

Of the 48 cases, the 2DP accurately recognized fluid amounts between 500 and 2000 ml in 39 (81%), 500 ml in 20 (57%) of the cases, and > 2000 ml in just 1 (7%) of the cases.

A Subjective Assessment

A skilled sonographer will visually inspect the uterine cavity and determine whether the amniotic fluid volume is low, normal, or high without using any quantitative methods.

In a study comparing the subjective assessment of amniotic fluid volume in singleton pregnancies with multiple ultrasound estimates, including the SDP, the 2DP, and the AFI, the investigators found that the subjective assessments were just as accurate as the objective ultrasound measurements in classifying fluid volumes as oligohydramnios, normal, and polyhydramnios.

Amniotic fluid volumes in each sac of a twin pregnancy, the volume validated by dye-dilution procedures, were found to be identical in a study comparing subjective and objective judgments.

The success rate for low-volume recognition ranged from 7% to 29%.

The Importance Of Assessing The Amniotic Fluid In Twins

Ultrasound of Amniotic Fluid

Amniotic fluid volume is measured as part of the fetal anatomic survey in singleton pregnancies, and it is measured as part of antenatal testing in at-risk pregnancies.

This is especially true in twin pregnancies, which have a higher risk of perinatal morbidity and mortality.

Normal fluid volumes must be known before an abnormal fluid volume in a twin pregnancy can be detected.

The fact that only one study in the literature has precisely calculated the amniotic fluid volume in each sac of a twin pregnancy using the gold standard dye-dilution technique demonstrates the difficulties of doing so.

As a result, the amniotic fluid volume has been estimated using ultrasound measurements such as the AFI, SDP, and 2DP methods.

Furthermore, subjective judgment - visualization without measurement - has been found to be as accurate as dye-dilution ultrasound measures.

Unfortunately, comparing ultrasound measurements to predicted fluid volumes is the only way to establish which ultrasound measurements better detect oligohydramnios, normal amniotic fluid volume, and polyhydramnios.

When employing an ultrasound estimate of amniotic fluid volume, it is obvious that volumes must be measured in each individual sac.

The failure of the total AFI, which neglected membrane location, highlights the significance of measuring or subjectively appraising each amniotic sac.

People Also Ask

What Is Normal AFI For Twins?

Among twins A and B, the average AFI was 136 (95% CI: 133-139) and 137.0 (95% CI: 134-140), respectively.

Do You Have More Amniotic Fluid With Twins?

If you're pregnant with twins or multiples, you're at risk for high fluid levels since they create more fluid than a single kid.

What Is Borderline AFI?

Borderline AFI was defined as an AFI greater than 5 cm and less than 8 AFI. Cesarean section was performed due to the pregnancy's results, which included low birth weight, umbilical cord blood pH at term, meconium-stained amniotic fluid, a 5-min Apgar score of 7, and NICU hospitalization.


Between pregnancies with borderline and normal AFI, there are substantial disparities for bad pregnancy outcomes, such as Cesarean section because of unsettling fetal heart rate, birth weight less than the 10th percentile for gestational age, low 5-minute Apgar score, and low umbilical artery pH.

The current study's findings show that perinatal outcomes could be unfavorable in pregnancies with borderline AFI.

These pregnancies should therefore be closely monitored by regular fetal assessment, intrapartum monitoring, and neonatal care.

There is still a need for more research because there is no universal agreement on fetal testing, prompt intervention, and intrapartum fetal monitoring.

In this category of pregnancies, studies using color Doppler measurement of cerebroplacental ratio are useful.

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About The Authors

Karan Emery

Karan Emery - I'm a research scientist interested in learning more about how neural activity influences and shapes human behavior. Project design and management, data analysis and interpretation, and the creation and implementation of testing tools are among my specialties. I enjoy coming up with new ideas and coming up with practical solutions to issues that are widely applicable. My colleagues would describe me as a driven, resourceful individual who maintains a positive, proactive attitude when faced with adversity. Currently, I’m seeking opportunities that will allow me to develop and promote technologies that benefit human health. Specific fields of interest include data analytics, biotechnology, and pharmaceuticals.

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