Amniotic Fluid - Assessing Its Volume In Twins
Because twin pregnancies have a higher risk of spontaneous abortions, malformations, low birth weight neonates, and preterm deliveries than singleton pregnancies, assessing amniotic fluid volume during the fetal anatomic survey, growth assessment, and as a component of antenatal testing is even more important in twin pregnancies.
Pregnancies with gestational diabetes, hypertension/preeclampsia, acute fatty liver, and placental abruption are also more likely.
Normal amniotic fluid volume must be defined before abnormal amniotic fluid volume in twin pregnancies can be assessed. The only study that looked into this issue looked at amniotic fluid volume in diamniotic twin pregnancies between 27 and 38 weeks.
The volumes per individual amniotic fluid sac ranged from 155 to 5430ml, with a mean of 877ml, similar to that seen in singleton pregnancies, according to the researchers. Charles and Jacoby's dye-determined volume techniques were used to determine the volumes of each sac.
This method is invasive and necessitates laboratory assistance, making it unsuitable for daily amniotic fluid volume measurements. The ultrasound estimation of amniotic fluid volume was developed in response to the need for a more viable alternative.
The amniotic fluid index, the single deepest pocket (SDP), the two diameter pocket (2DP), and the subjective assessment of the amniotic fluid volume have all been used to estimate the amniotic fluid volume in twins using ultrasound.
The technique described by Phelan is commonly used to estimate the amniotic fluid volume in singleton pregnancies. The umbilicus transversely divides the uterus into upper and lower quadrants, and the linea nigra divides the right and left halves. The largest vertical pocket of fluid within each quadrant without an aggregate of cord or foetal small parts is measured in centimetres with the ultrasound transducer held perpendicular to the floor.
For a pocket to be measurable, it must have a horizontal measurement of at least 1cm all the way around. A number of researchers have used the summated AFI to calculate the amniotic fluid volume in twin pregnancies. By dividing the abdomen into four quadrants, this estimate is made in a similar way to the technique used in singleton pregnancies. The summated AFI is calculated by measuring the largest pocket in each quadrant and adding it together.
When calculating the summated AFI, membrane placement is not taken into account. When dye-dilution techniques were used to test the accuracy of the summated AFI, the summated AFI correctly identified 94% of twin pairs as having normal amniotic fluid volume, despite the fact that only 52% of twin pairs had normal volumes.
To determine the amniotic fluid volume in twins, a number of other techniques have been used under the category of AFI. Gerson et al. calculated the amniotic fluid volume by locating the dividing membrane between the twins, dividing the amniotic fluid sac into upper and lower halves using the foetal diaphragm, measuring the largest pocket free of umbilical cord in centimetres, and summing the measurements to produce a two-quadrant AFI.
Hill et al. identified each foetus and its surrounding amniotic sac, then measured the largest vertical pocket in each of the four quadrants of the sac surrounding the foetus, using the foetus as the vertical axis in the sac. The AFI was the summation of these 4 measurements. A dye-determined amniotic fluid volume has not been used to test either of these methods.
The technique described by Chamberlain is usually used to estimate the amniotic fluid volume in singleton pregnancies. The vertical and transverse diameters of the largest pocket of fluid were measured by placing the ultrasound transducer at a right angle to the uterine contour. At a right angle to the depth of the measurement, the width of the largest pocket of amniotic fluid was determined. The width of the pocket was less than ≥1cm in all cases except those with severe oligohydramnios (vertical pocket of <1 cm).
Severe oligohydramnios was defined as a depth of less than one centimeter, oligohydramnios was defined as 1-2 centimeters, and normal was defined as a depth of more than two centimeters to eight centimeters. The largest vertical pocket of 8 was identified as polyhydramnios in a companion article published in the same journal.
Many clinicians now measure the largest pocket without the umbilical cord or small parts of the foetus, or only if their appearance is temporary. The separating membrane is located and the largest vertical pocket of amniotic fluid volume in each sac is measured in centimetres when estimating amniotic fluid volume using the SDP in twins.
Oligohydramnios is defined as a measurement of less than 2 cm, normal is 2-8 cm, and hydramnios is greater than 8 cm. These are the same measurement thresholds used in singletons. The SDP correctly classified fluid volumes between 500 to 2000 ml in 47 of 48 cases (98%), but poorly identified volumes less than 500 ml in 1 of 35 cases (3%), and more than 2000 ml in 0 of 7 cases (0%).
The 2-dameter pocket technique is defined as the vertical measurement multiplied by the horizontal measurement of the largest identified pocket of amniotic fluid in singleton pregnancies. Oligohydramnios was defined as a two-diameter pocket with a diameter of less than 15 cm, normal fluid volume was defined as 15 – 50 cm, and polyhydramnios was defined as a two-diameter pocket with a diameter of more than 50 cm.
The separating membrane and the largest pocket of fluid in each sac without foetal small parts or the umbilical cord are identified in twin pregnancies, and the horizontal measurement is multiplied by the vertical measurement. Twins are given the same values as singletons for oligohydramnios, normal, and polyhydramnios. The 2DP correctly classified fluid volumes between 500 to 2000 ml in 39 of 48 cases (81%), identified volume 500 ml in 20 of 35 cases (57%), and > 2000 ml in 1 of 7 cases (14%).
The subjective assessment of amniotic fluid volume involves an experienced sonographer visualizing the fluid volume and estimating whether it is low, normal, or high based on visualization alone, without taking any measurements.
The subjective assessments were similar in accuracy to the objective ultrasound measurements in classifying volumes of fluid as oligohydramnios, normal, and polyhydramnios, with those volumes validated by dye-determination techniques in an investigation evaluating the subjective assessment of amniotic fluid volume in singleton pregnancies with multiple ultrasound estimates including the SDP, 2DP, and the AFI.
A similar study was carried out in twin pregnancies, and the subjective and objective evaluations in the identification of amniotic fluid volumes in each sac of a twin pregnancy, the volume validated by dye-dilution techniques, were found to be similar. Low volume recognition was poor, with 7-29% of the time.
In singleton pregnancies, amniotic fluid volume is measured as part of the foetal anatomic survey, and in at-risk pregnancies, it is measured as part of antenatal testing. That evaluation becomes even more important in twin pregnancies, which have a higher risk of perinatal morbidity and mortality. Normal volumes must be identified in order to recognize an abnormal fluid volume in a twin pregnancy.
The difficulty of accurately calculating the amniotic fluid volume in each sac of a twin pregnancy using the gold standard dye-dilution technique is demonstrated by the fact that only one study in the literature has done so. As a result, various ultrasound measurements, such as the AFI, SDP, and 2DP techniques, have been used to estimate amniotic fluid volume.
Furthermore, the subjective assessment - visualization without measurement - has been shown to be as accurate as ultrasound measurements using the dye-dilution technique. Unfortunately, the only way to confirm which ultrasound measurements better identify oligohydramnios, normal amniotic fluid volume, and polyhydramnios is to compare the ultrasound measurement to a calculated volume of fluid.
It is clear that volumes must be measured in each individual sac when using an ultrasound estimate of amniotic fluid volume. The failure of the summated AFI, which ignored membrane placement, demonstrates the importance of measuring or subjectively assessing each amniotic sac.
The mean amniotic fluid volume per amniotic sac ranged from 155 mL to 5430 mL with a mean ± 1 SD of 877 ± 860 mL. Hence, the amniotic fluid volume in twins was similar to or slightly greater than the reported volume in single pregnancies.
A fluid pocket of at least 8 cm was found in 9.8% of twin pregnancies at 26 to 32 weeks' gestation, but this did not indicate any pathologic conditions. Fluid volume in twin pregnancy can be assessed using either AFI or measurement of the deepest fluid pocket in each sac.
Monoamniotic twins are identical twins who share an amniotic sac (also known as the "bag of waters"), a fluid-filled sac that holds the baby during pregnancy. Identical twins have their own amniotic sacs in most cases. Monoamniotic twins are a rare type of "monochorionic" twins in which the placenta is shared.
One twin (dubbed the "donor" twin) gives the "recipient" twin too much blood. Because of the extra blood, the recipient's kidneys produce more urine, resulting in a large bladder. Polyhydramnios (excess amniotic fluid), prenatal heart failure, and hydrops (excess swelling) are all possible outcomes.
In the third trimester of pregnancy, the volume of amniotic fluid in normal diamniotic twin pregnancies is similar to the volume in singleton pregnancies. Although the volume of amniotic fluid in each sac can be precisely measured, the techniques for doing so are impractical for everyday use.
Because it does not take membrane placement into account, the summated AFI is unable to distinguish between individual low and high sac volumes and should not be used to estimate amniotic fluid volume. The other methods for determining individual sac volume, such as AFI, SDP, 2DP, and subjective assessment, can all reliably identify normal amniotic fluid volume, but not oligohydramnios or polyhydramnios. To estimate the amniotic fluid volume in diamniotic twin pregnancies, many health care providers currently measure the single deepest pocket of each sac.