Ultrasound and Twins: Copy Text

Methods of determining twin placentation by ultrasound

Determining Chorion and Amnion Status

Recently, noninvasive prenatal determination of twin zygosity by maternal plasma DNA sequencing has been reported as feasible. However, for the foreseeable future, ultrasound will remain the primary determinant of zygosity and amnion and chorion status in defining the type of multiple gestations.  Both transvaginal ultrasound and increasingly improved imaging modalities make this possible. The following videos demonstrate the concepts related to placentation in twin and multiple pregnancies. Diamniotic Dichorionic Twins
Above.  Schematic of the evolution of diamniotic dichorionic twin gestation.  Two separate sperm fertilize two separate ova, normally generating two separate pregnancies within the uterine cavity.  The placentas of each pregnancy have one amnion and one chorion (diamniotic dichorionic placentas).  This type of diamniotic dichorionic twinning occurs in 80% of all such pregnancies.  However, 20% of the time diamniotic dichorionic placentation can occur in one egg twins (monozygotic) if division occurs within the first three days of fertilization.  When the division of a single egg occurs within the first three days after fertilization, the twins will be “identical.”
Above.  Evolution of monochorionic twin gestations.   A single sperm fertilizes a single ova.  The placental type which evolves depends upon the timing of the cleavage of the fertilized ova. If the fertilized egg divides within the first 3 days, diamniotic dichorionic twins occur in the presence of a single ovum.
Above. Diamniotic dichorionic twins if a single egg spits within the first 3 days after fertilization. If the split occurs 4 to 8 days after fertilization, diamniotic monochorionic twins may occur (This is the most common type of “identical twins”).
Above. Diamniotic monochorionic twins. If the split occurs 9 to 13 days after fertilization, monoamniotic monochorionic twins may occur (The twins reside in a single sac and are vulnerable to cord entanglement).
Above. Monoamniotic monochorionic twins
Abover image. If the split occurs at greater than 13 days after fertilization, conjoined twins may occur.
Above.  Schematic demonstrating dividing membrane arrangement in diamniotic dichorionic twin gestation.  Each twin contributes one amnion and one chorion to the dividing membrane.  The membrane nearest to the fetus is the amnion.  Therefore, the membrane is thicker than in diamniotic monochorionic twins and the layers are composed of ACCA (amnion-chorion-chorion-amnion).
Above.  Placenta of diamniotic dichorionic twins.  Note twin peak sign and relatively thick dividing membrane.  Fetal genders are discordant. Diamniotic Monochorionic Twins
Above.  Schematic demonstrating diamniotic monochorionic twin gestation.  In this type of twinning, the placenta is composed of 2 amnions (AA) which comprise the thin dividing membrane; 1 chorion encircles the gestational sac and does not contribute to the dividing membrane.
Above.  Schematic demonstrating the potential vascular arrangement in diamniotic monochorionic twins.  Virtually all such twin pregnancies have vascular communications between the placentas.  These arrangements may be artery to vein, artery to artery, or vein to vein.  The number and arrangements of these vascular communications contribute to potential complications such as twin-twin transfusion syndrome and fetal growth restriction.
Above.  Diamniotic monochorionic twins.  Note thin dividing membrane with T insertion into the placenta.  The placenta is fused and fetal genders are concordant.  The membrane is thin but visible.
Above.  Diamniotic monochorionic twin gestation with discordant fetal growth and polyhydramnios in the recipient (R) sac.  Note smaller donor (D) twin and the thin monochorionic dividing membrane.
Above.  Diamniotic monochorionic twin gestation with discordant fetal growth and polyhydramnios in the recipient (R) sac.  Moderate fetal activity is noted in this recipient fetus. Diamniotic Monochorionic Twin Gestations In diamniotic monochorionic placentas, there is one chorionic ring, a thin dividing membrane, a T-insertion of the dividing membrane, a fused placenta, and an embryo in each sac. Above.  18 weeks gestation.  Note thin dividing membrane with T extension to the placenta barely visible, characteristic of the diamniotic monochorionic placenta.  The placenta is fused and the fetal gender is concordant.
Above. Note fused placenta and thin dividing membrane.  The fetuses are of unequal size suggesting early growth restriction.  Fetal genders are concordant.
Above. again, note thin dividing membrane suggests a diamniotic monochorionic twin placenta. Monoamniotic Monochorionic Twins
Above.  Monoamniotic monochorionic twin pregnancy, mid-trimester.  Color Doppler demonstrates multiple umbilical cord loops.  There is no discernible dividing membrane and the fetus is normally active.
Above.  Monoamniotic monochorionic twins.  Color Doppler defines multiple segments of the umbilical cord within the gestational sac which contains no dividing membrane.  The placenta is fused and the umbilical cord insertions into the placenta are in close approximation.  Fetal gender is concordant.
Above.  Monoamniotic monochorionic twin pregnancies with similar findings as noted above.
Above.  Color Doppler, monoamniotic monochorionic twin pregnancy, mid-trimester.  The amount of amniotic fluid appears to be reduced and fetal extremities are adjacent to the multiple umbilical cord segments.

Role of ultrasound in the follow-up of twin gestation

Most guidelines recommend ultrasound screening of dichorionic twins every four weeks and monochromic twins every two weeks beginning at 16 weeks.  Follow-up will be discussed for specific twin complications reviewed elsewhere in the curriculum, and the role of ultrasound will be defined in detail for each complication. Ultrasound follow-up is critical to an understanding and evaluation of twin gestations and their potential complications.
Above. The twin complication rate and the death rate is greater than that for singleton pregnancies and is affected by the occurrence of premature births.
Above is listed the potential complications for multiple gestations, inclusive of twins. The role of ultrasound can be broadly categorized into those related to specific abnormalities due to the type of twin placentation, while others are attributable to over-distension of the uterus, and traumatic births. Specific complications such as fetal growth disturbance, twin to twin transfusion syndrome, conjoined twins and twin reversed arterial perfusion will be reviewed elsewhere in this curriculum. Aneuploidy Risk In dizygotic pregnancies, there are two separate fetuses, each with its own chromosome complement. The risk for aneuploidy is, therefore, greater than in a singleton pregnancy. Each fetus in a dizygotic pregnancy is assigned a risk, while the risk of aneuploidy in a monozygotic pregnancy is similar to the age-adjusted risk for a singleton pregnancy. Any screening method which yields an increased risk for aneuploidy should be followed up with an ultrasound.  The nuchal translucency (NT) may allow a specific risk for an individual fetus in a dizygotic twin pregnancy and appropriate follow-up is established according to the specific condition. Congenital Anomalies Overall, twins have a greater number of congenital anomalies compared to singletons and twins have twice the prevalence of cardiovascular anomalies compared to singletons, particularly among like-sex (monozygotic) twins. Detection of congenital malformations in twins is possible during the first trimester, especially those of the cranial vault, midline brain, and abdominal wall defect. The most common malformations are cardiac and renal.  Appropriate follow-up is established according to the specific condition anomaly. Screening for Preeclampsia Uterine artery Doppler screening is known to be predictive of preeclampsia in singletons. In twins, using uterine artery pulsatility index, certain maternal factors, mean arterial pressure, and placental growth factor, the detection rate for preeclampsia is high.  Screen-positive women with twins may be treated with low-dose aspirin but further data is necessary to determine if the incidence of preeclampsia is reduced with this treatment regimen. Preterm Labor Prediction of preterm labor in twin gestations is possible with transvaginal ultrasound. However, at present, the data appears to be limited on knowing the effect of ultrasound on preventing preterm birth in twins.
Above is a summary of the placental type,  incidence, major complications, and mortality among various twin types.

Role of ultrasound in the serial evaluation of discordant twins

Above.  C-section birth of Twin A, growth-restricted twin (EFW=estimated fetal weight, AC=abdominal circumference). Above.  Left. Discordant twins at birth, term.  Right.  Pre-term. Growth restriction occurs in approximately 25% of multiple gestations. Twins represent about 1% of pregnancies but account for 12 % of early neonatal deaths and 17% of all growth-restricted infants. [1] Triplets and other higher-order multiple gestations also account for a significant number of growth-restricted fetuses.

Evaluation of Fetal Growth

Above.  Examples of longitudinal assessment of fetal growth in twins over time (EFW=estimated fetal weight, AC=abdominal circumference).  In multiple pregnancies, two methods are used to evaluate fetal growth: growth discordance (differences in fetal weight frequently expressed as a percent) and longitudinal assessment of the weight or growth for the individual fetus. Degrees of Discordance Because there is a relationship between severe growth discordance and neonatal developmental delay, birth weight discordance is considered important.  In general, 15% or more birth weight difference between twins is considered mild discordance, and ≥ 25% birth weight difference is classified as severe discordance. If the birth weight discordance is ≥ 25%, the perinatal death rate is increased by a factor of 2.5, and risk of fetal death is increased by a factor of 6.5. Neonatal Morbidity and Mortality Neonatal morbidity is best predicted by individual birth weight percentiles of < 15th and < 10th, respectively.  Outcomes such as neonatal death, congenital anomalies, small for gestational age (SGA), and periventricular leukomalacia are defined by a birth weight difference of 30%.  Estimated fetal weight discordance of 25% or more had a sensitivity of 55% for predicting actual birth weight discordance. Antepartum Diagnosis The antepartum diagnosis of discordance was correct in 4 of 5 cases.  However, discordance may be over-diagnosed in almost 20% of cases, and almost half of the significantly discordant twin pairs can be missed.  A number of authors recommended multiple individual parameter assessments (individualized growth assessments) for the twin fetus suspected of growth restriction. This approach simplifies interpretation of clinical data, allowing the fetus to be followed over time and permitting focused surveillance.

Sonographic findings of Monoamniotic Monochorionic Twin Gestation

No dividing membrane suggests monoamniotic monochorionic twins.  Color Doppler demonstrates umbilical cords from each of the fetuses to be entangled. Overall, cord entanglement is the principal cause of mortality in monochorionic monoamniotic twins, and intensive fetal surveillance and early cesarean delivery may improve outcomes for monochorionic monoamniotic twins The insertion of the umbilical cord from each twin into the placental surface is frequently of a short distance from each other. Above.  Early in the first trimester, monoamniotic monochorionic twins will demonstrate only one yolk sac.  Typically, the number of yolk sacs corresponds to the number of amnions.  In this schematic example, the single amnion is the inner layer of the membrane closest to the fetus, while the single chorion is the outer layer of the membrane.  Note the placental umbilical cord insertions from each fetus are in close proximity to each other; between the placental cord insertions are numerous vascular connections.  The latter likely contributes to growth which is commonly similar in each fetus. Above.  This is a placenta from a monoamniotic monochorionic pregnancy.  Again, note the short distance between the umbilical cord placental insertions and the numerous vascular connections between these insertion sites. Above.  Mid-trimester color Doppler image of the umbilical cords in a monoamniotic monochorionic pregnancy.  It is difficult to distinguish umbilical cord knots.  Note there is no dividing membrane.  The fetal genders are the same, and there is a single placental mass. In summary, between 9 and 12 weeks gestation, the dividing membrane should be visible on transvaginal ultrasound.  A thick dividing membrane suggests a diamniotic placenta and a thin dividing membrane suggests a monochorionic placenta.  No dividing membrane suggests monoamniotic monochorionic twins.  Measure the CRL for each twin.  The median CRL discrepancy was 11.9% between twins with fetal growth restriction while the median CRL discrepancy was 3.8% for the TTTS group and 3.5% for the control group. 2.  Count the number of yolk sacs.  The yolk sac and the amnion develop temporally before 6 weeks gestation but the sequence of development is not clear. In MC twin pregnancies, the number of yolk sacs determined between 6 and 9.5 weeks often correlates with the amnion status. (1 yolk sac = 1 amnion).  That is, if 2 yolk sacs are seen, a diamniotic monochorionic pregnancy can be assumed while the presence of 1 yolk sac was assumed to represent a monochorionic monoamniotic twin gestation.  Recently, that observation has been challenged since 2 yolk sacs have been associated with monoamniotic monochorionic pregnancies.  Others reported similar observations.  In addition, a single yolk sac occurs in diamniotic monochorionic pregnancies in 3 of 20 or 15% of cases. Conclusions:  Among monochorionic placentas observed during the early first trimester, 2 yolk sacs are usually but not always associated with the presence of diamniotic monochorionic gestation.  The presence of a single yolk sac may also be associated with a diamniotic monochorionic pregnancy while the presence of 2 yolk sacs does not exclude the possibility of monoamniotic monochorionic pregnancy. 3.  Amnion status can be determined by counting the number of embryonic heartbeats within each gestation sac.  Two gestational sacs and 2 separate heartbeats would suggest diamniotic status. 4.  Measure the nuchal translucency (NT) for each twin.  The nuchal translucency measurement above the 95th percentile is correlated with fetal aneuploidy and certain malformations.  Among a large series of monochorionic twins (N=242), median inter-twin NT discrepancies were not different between those who developed TTTS and controls.

Mid-trimester and Third Trimester

1.  Determine the number of placentas.  If the placentas are physically separate (i.e., posterior and anterior wall of the uterus), diamniotic dichorionic placentas are most likely. 2.  Determine if the placentas are fused.  If the placentas are fused, any type of gestations are possible, and then assess membrane characteristics. 3.  If the membrane is thick, diamniotic dichorionic placentas are most likely.  If the membrane is wispy and thin, diamniotic monochorionic placentas are most likely.  If there is no dividing membrane, monoamniotic monochorionic placentas are most likely. 4.  Twin peak sign is a portion of tissue extending into the base of the inter-twin membrane and is associated with diamniotic dichorionic gestation. 5.  If in doubt, count the number of layers within the dividing membrane.  If greater than 2, diamniotic dichorionic placentas are most likely. 6.  Determine gender.  If the genders are different, diamniotic dichorionic placentas are most likely.

Accuracy in Ultrasound Determination of Chorion and Amnion Status

For the determination of chorion status and amnion status in twin pregnancies, transvaginal ultrasound at between 7 and 9 weeks gestation is comparable in accuracy (97%) to an ultrasound scan performed at 11 to 14 weeks gestation. Sonographers and perinatologists scanning the same patients have a rate of discordant results of less than 1% among diamniotic dichorionic pregnancies and a rate of discordant results of 5.5% among diamniotic monochorionic pregnancies Based upon placental pathology, correct prediction of chorion status was made in all but 2 of 82 patients scanned at between 5.5 and 26 weeks gestation. Recent Observations Recently, the diagnostic value for birth growth discordance (BGD) has been challenged.  This review (19) suggests there is insufficient evidence to support such indices as the only measure for clinical decision-making in twin gestation. The diagnostic accuracy of other measures including amniotic fluid index and umbilical artery Doppler resistant indices in combination with ultrasound for clinical intervention requires further evaluation. (19) In addition, other methods to assess fetal growth have shown promise to more accurately define growth parameters. (24)  An individualized growth assessment: conceptual framework and practical implementation for the evaluation of fetal growth and neonatal growth outcome has been proposed and an Individualized Growth Assessment Program is available. (24)

Ultrasound Estimation of Amniotic Fluid in Twins

As noted previously, because of the increased risks for adverse outcomes in twins, proper assessment of amniotic fluid volume is important. There are a number of conditions that are associated with either an increase or decrease in amniotic fluid
Above. Twin conditions in which the assessment of amniotic fluid volumes is useful. Amniotic Fluid Volume in Twins Individual amniotic fluid indices have been obtained in twin pregnancies, and the amniotic fluid values compare favorably with those of singleton gestation.  Dye dilution methods have determined amniotic fluid volumes in twin pregnancies but this method is not practical for clinical use since it is an invasive technique. There are, however, a number of other methods which have been explored for this purpose in twins.
Above. Methods for amniotic fluid volume determination in twins. The amniotic fluid index was originally described for singleton pregnancies.
Above. For the amniotic fluid index (AFI), the uterus is divided into 4 quadrants.
Above. In the AFI, the transducer is held perpendicular to the floor. The sum of the 4 pockets of fluid are added and an index is derived. Normal values are provided in the year 1 curriculum. The amniotic fluid index and the two diameter pockets (2DP) are problematic for the estimation of amniotic fluid volume in twins. The AFI is poor at estimating oligohydramnios in twins, and the two diameter pockets (2DP) method correlates poorly with the gold standard dye method.  In addition, the subjective assessment of fluid in twins poorly recognizes oligohydramnios and polyhydramnios in twins. Single Deepest Pocket (SDP) The single deepest pocket of fluid is currently the most commonly used method to assess amniotic fluid volumes in pregnancy.
Above are the criteria for the SDP as reported for singleton pregnancies.
Above are the values for the SDP as reported for singleton pregnancies. Twin Amniotic Fluid Assessment In summary, for twin assessment, a single deepest pocket of fluid (SDP) is the most universally used method. In twins, the separating membrane is located and the largest vertical pocket of amniotic fluid volume in each sac is measured in centimeters. A measurement of ≤ 2 cm is classified as oligohydramnios, 2-8 cm as normal, and > 8 cm as hydramnios,
Above are cutoff values for SVP of fluid for twin pregnancies.
Above. Example of a single deepest pocket (SDP).