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AFI In Twin Pregnancy Radiology - How It Can Be Measured

The use of ultrasound to measure the amount of amniotic fluid has grown in importance as a way to predict fetal health.

A subjective assessment, measuring the depth of the deepest umbilical cord-free pocket, and using the semi-quantitative amniotic fluid index (AFI) have all been used to figure out how much amniotic fluid is in the womb.

Recently, the AFI has become the method most often used to measure the amount of amniotic fluid.

Using vertical and horizontal lines through the umbilicus, the uterus is divided into four quadrants for the purpose of measuring an AFI in a singleton gestation.

Few studies have used the AFI in twin pregnancy radiology, despite the fact that amniotic fluid evaluation is a crucial part of perinatal treatment, particularly for twin gestations.

Instead of an index unique to each twin fetus, each of these papers describes a total pregnancy AFI.

It's possible that the difficulty in determining common landmarks for the quadrants is the reason why there aren't many reports linking AFI to specific babies in twin gestations.

Because the membrane and fetal position can change, it can be hard to find these quadrants.

How Is The Normal Amniotic Fluid Volume Estimated?

The amniotic fluid index is found by adding up the diameters of the largest fluid pocket in each quadrant that is not filled with the umbilical cord or other parts of the fetus.

Normal AFI ranges from 7 to 25 cm.

Additionally, the size of each fluid pocket should range from 2 to 8 cm.

Oligohydramnios means that there is not enough amniotic fluid, and polyhydramnios means that there is too much amniotic fluid.

Clarius Feature: OB Amniotic Fluid Index

Semi-Quantitative Ultrasound Assessment Of Amniotic Fluid Volume

In the clinic, the AFI and MVP are the two most common semiquantitative ways to measure AF volume, which is also known as the single deepest vertical pocket.

Since Moore and Brace first came up with the idea in 1998, many studies have looked at the link between AF volume (both AFI and/or MVP) and dye-determined or directly measured AF volume.

Unfortunately, the sensitivities range from 71% to 98%, and the sonographic estimates of normal AF volume may not always match up well with the direct or dye-determined methods.

The sensitivities range from 6.7% to 27%, which is low compared to dye-determination or directly measured volumes for finding oligohydramnios.

In 1987, Phelan and his colleagues came up with the AFI for full-term pregnancies.

When the transducer is in a sagittal plane perpendicular to the floor, four vertical quadrants are added together.

Following the development of gestation-specific normative values for AFI, this measuring technique was subsequently expanded to include second and third-trimester pregnancies (16 - 42 weeks gestation).

In the MVP method, which is also widely used, a 2-cm cutoff is the most commonly accepted clinical threshold for telling the difference between low and normal AF.

An AFI of 5 cm or less or an MVP of 2 cm or less is typically regarded as the ultrasonography threshold for low AF volume since these values have been linked to a higher risk for unfavorable perinatal outcomes.

Even though it is less clear, the top limit of AFI to define polyhydramnios is often larger than 24 cm (or MVP > 8 cm).

Measurement Of Amniotic Fluid Index

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 placed along the longitudinal axis of the mother's abdomen and kept perpendicular to the floor while scanning.

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

Even though it loses its ability to predict at the extremes, the AFI is pretty close to 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 the pregnancy, the mean AFI is between 12 and 14 cm, but 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.

MME TV: How do I get an AFI?

A Case Study Of Ultrasound Determination Of Amniotic Fluid Indices

A modified AFI method was used to check the amniotic fluid of all twin-pregnant women who had routine ultrasounds to check on fetal growth between January 1991 and December 1994.

Individual fluid determinations need the ability to see a separating membrane between the twins.

The place of each fetus was written down based on where it was in the uterus in relation to the membrane.

The diaphragm of each fetus was identified in order to calculate the modified AFI.

Each fetal sac was divided into two parts by the fetal diaphragm.

Above and below each fetal diaphragm, the size of the largest fluid pockets that were not connected to the umbilical cord was measured in cm.

When the two measurements for each fetus were added together, the AFIs for babies A and B were found.

Using a 3.5 MHz or 5.0 MHz abdominal transducer, an Acuson 128 ultrasound machine was used for all ultrasonography measurements of AFI.

The ultrasounds were done by two sonographers, and one of four perinatal doctors looked at the results.

At each evaluation, the method was able to locate a twin-specific AFI.

The trial was open to people whose gestational age could be confirmed by ultrasonography by 15 weeks.

Patients were disqualified if there was evidence of discordance (more than 25% difference in birth weight), the twins were monoamniotic, there were congenital anomalies, or the mother had been diagnosed with diabetes or hypertension.

The Results

There were 128 moms in the original dataset.

There were 15 mothers who developed diabetes or hypertension, 11 babies who had birth weight discrepancies, and three children who were lost to follow-up.

37 of the 99 available mothers were excluded because they had only one observation.

This research is based on 216 datasets and 62 mothers.

Thirteen women had two observations in these datasets; 19 mothers had three; 17 mothers had four and thirteen moms had five.

What Does The Case Study Mean For AFI?

A straightforward and useful AFI for separate twin-gestation fetuses was established.

The method is very similar to the one used for singleton pregnancies, but it gets around the problem of finding landmarks (sac location, fetal position) that kept the singleton AFI from being fully useful for twin pregnancies.

In the case of a twin pregnancy, the AFI is calculated by adding the measurements of the deepest pocket above and below the diaphragm of each fetus.

This lets us get a rough idea of how much amniotic fluid is around each twin fetus between weeks 15 and 36 of a twin pregnancy.

Three prior studies have looked at the amount of amniotic fluid present during twin pregnancies.

Each of these terms describes an AFI that happens during both of the twins' pregnancies.

As of now, research does not give the doctor enough information to know if the amniotic fluid around one of the fetuses in a twin pregnancy is enough or not.

Intriguingly, all of these studies showed that total pregnancy AFI peaked early in the third trimester, much like the current study did.

This study also shows that there is a correlation between identical twins' AFIs.

Two AFIs are produced from a single set of twins, one of which is larger than the other.

A Pearson location correlation demonstrates that each fetus's AFI occupies a location on its own curve that is connected to the place of the AFI of its sibling if the larger observations are pooled separately from the smaller observations and regression is constructed.

This observation could be used to figure out if there is a problem with the amount of amniotic fluid in a twin pregnancy.

Possible indication of congenital abnormalities, transfusion between twins, etc.

A pregnant woman showing a sign of two with her fingers on her stomach
A pregnant woman showing a sign of two with her fingers on her stomach

The Importance Of Assessing The Amniotic Fluid In Twins

Amniotic fluid volume is assessed during the fetal anatomic survey in singleton pregnancies and during prenatal testing in high-risk pregnancies.

Pregnancies with multiples, like twins, increase the likelihood of perinatal morbidity and mortality.

Therefore, this assessment is especially crucial in these cases.

The only way to spot an abnormal fluid volume in a twin pregnancy is to be familiar with the normal range.

Only one study has attempted to calculate the amniotic fluid volume in both sacs of a twin pregnancy using the gold standard dye-dilution technique.

Therefore, other ultrasound metrics have been employed to quantify amniotic fluid volume.

These include the amniotic fluid index (AFI), the systolic fetal index (SDP), and the two-dimensional fetal positioning (2DP).

Ultrasound measurements utilizing the dye-dilution method have been demonstrated to be as accurate as subjective judgment.

Unfortunately, the only way to figure out which ultrasound measurements are more accurate for oligohydramnios, normal amniotic fluid volume, and polyhydramnios is to compare them to a predicted volume of fluid.

When using an ultrasound to estimate the amount of amniotic fluid, it's clear that each sac's volume must be measured.

The fact that AFI didn't take into account where the membrane was shown how important it is to take precise measurements or have an expert look at each amniotic sac.

People Also Ask

What Is Normal AFI For Twins?

In the case of twin babies, a study recorded the mean AFI of 136 (with a 95% confidence interval of 133-139) for twin A and the mean AFI of 137.0 (with a 95% confidence interval of 134-140) for twin B.

What Is The Normal Range Of Amniotic Fluid Index?

According to the conventional method of evaluation, an amniotic fluid index of 5 cm to 25 cm is considered normal.

Oligohydramnios is defined as having a length of less than 5 cm, whereas polyhydramnios is defined as having a length of greater than 25 cm.

How Can The Amniotic Fluid In Twins Be Increased?

Increasing the amount of water you drink, taking supplements, getting enough rest, starting the delivery process earlier, and amnioinfusion are some of the strategies that have been suggested for elevating amniotic fluid levels.

What If AFI Is High?

Premature labor, premature contractions that lead to an early delivery, premature placental detachment from the uterine wall, and premature rupture of the amniotic sac are all potential risks associated with premature labor.


Even though it is possible to get accurate measurements of how much amniotic fluid is in each sac, the ways to do so are not useful in everyday life.

Since it doesn't take into account where the membrane is, the summated AFI can't tell the difference between sacs with low and high volumes, so it shouldn't be used to estimate how much amniotic fluid there is.

None of the other ways to estimate the size of each sac can reliably find oligohydramnios or polyhydramnios, but they can reliably find a normal amount of amniotic fluid.

In twin pregnancy radiography, a lot of doctors and nurses now measure the single deepest pocket in each sac to get an estimate of the AFI.

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About The Authors

Suleman Shah

Suleman Shah - Suleman Shah is a researcher and freelance writer. As a researcher, he has worked with MNS University of Agriculture, Multan (Pakistan) and Texas A & M University (USA). He regularly writes science articles and blogs for science news website immersse.com and open access publishers OA Publishing London and Scientific Times. He loves to keep himself updated on scientific developments and convert these developments into everyday language to update the readers about the developments in the scientific era. His primary research focus is Plant sciences, and he contributed to this field by publishing his research in scientific journals and presenting his work at many Conferences. Shah graduated from the University of Agriculture Faisalabad (Pakistan) and started his professional carrier with Jaffer Agro Services and later with the Agriculture Department of the Government of Pakistan. His research interest compelled and attracted him to proceed with his carrier in Plant sciences research. So, he started his Ph.D. in Soil Science at MNS University of Agriculture Multan (Pakistan). Later, he started working as a visiting scholar with Texas A&M University (USA). Shah’s experience with big Open Excess publishers like Springers, Frontiers, MDPI, etc., testified to his belief in Open Access as a barrier-removing mechanism between researchers and the readers of their research. Shah believes that Open Access is revolutionizing the publication process and benefitting research in all fields.

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