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Ultrasonographic criteria for the prenatal diagnosis of placental chorionicity in twin gestations
Ultrasonographic criteria for the prenatal diagnosis of placental chorionicity in twin gestations Hung N. Winn, MD," Sandro Gabrielli, MD: E. Albert Reece, MD," J. Andres Roberts, MD," Carolyn Salafia, MD," and John C. Hobbins, MD" New Haven, Connecticut Thirty-two patients with uncomplicated twin pregnancies had ultrasonographic examinations for genetic amniocentesis, confirmation of twinning, or assessment of fetal growth. The dividing membranes between the fetuses were visualized, and the thickness of the membranes was measured. With a thickness of 2 mm used as a cutoff point, the accuracy in predicting monochorionic or dichorionic twinning was 82% and 95%, respectively. Prenatal assessment of these dividing membranes may be helpful in the management of twin gestations. (AM J OBSTET GVNECOL 1989;161 :1540-2.)
Key words: Twins, chorion, amnion, ultrasonography, prenatal diagnosis In the United States, the overall incidence of twinning is about one in 90 pregnancies. I Complications from twin pregnancies still contribute significantly to the overall perinatal mortality and morbidity. The incidence of morbidity is affected by the status of the placental chorion and amnion. Since monochorionic placentas almost always have some vascular anastomoses between the two fetal circulations, identification of placental chorion is useful in the management of a twin pregnancy, especially when it is complicated by growth discrepancy, fetal hydrops, or fetal death. Whereas the identification of different fetal genders or two separate placentas indicates dichorionic twinning,2 differentiation between monochorionic and dichorionic placentas must rely on other parameters when only one placenta or similar genders exist. Qualitative evaluation of the dividing membranes by ultrasonography has been attempted to determine the type of placental chorion.' The current study was undertaken, however, to quantitatively establish the so no graphic criteria for the prenatal diagnosis of chorion status in twin gestations.
Patients and methods This study was conducted at the Perinatal Unit of Yale-New Haven Hospital during the period January 1987 to January 1988. Indications for ultrasonographic From the DivislOn of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology," and the Department of Pathology,' Yale University School of Medlcl1te. Presented at the Thlrty-fifth Annual Meeting of the Society for G.vnecologie invest!gatlOn, Balt!more, Maryland, March 17-20, 1988 Reprint requests: Hung N. Winll , MD, Department of Obstetrics and Gynecology, Washington University Medical Center, 4911 Barnes Hospital Plaza, St. LOUIS, MO 63110. 6/6/162J5
1540
examination included confirmation of twin gestation, assessment of fetal growth, and genetic amniocentesis. The dividing membranes between the twins were evaluated as part of the ultrasonographic examination. After the membranes were well visualized and delineated, their thickness was measured between the outer borders, perpendicular to the long axis of the membrane segment (Fig. I), without prior knowledge of fetal gender or number of placentas. The segment of the dividing membranes used was as perpendicular to the ultrasonographic beams as possible to utilize the better axial resolution. The ultrasonographic scans were obtained with the Acuson-128 ultrasonography machine (Acuson, Mountinview, Calif) with 3.5 and 5 MHz transducers and an axial resolution of about 0.5 mm. Placental chorionic type was confirmed by histologic examination after delivery. The cutoff point of the membrane thickness measured by ultrasonography to differentiate between monochorionic and dichorionic placentas was retrospectively determined on the basis of results of the histologic examinations. Thirty-two twin pregnancies with dividing membranes evaluated both ultrasonographically and histologically formed the basis of this report. In four patients who had longitudinal ultrasonographic examinations of the dividing membranes, only one measurement for each patient was used to determine the cutoff point of membrane thickness (Tables I and II). The difference between the mean membrane thickness was analyzed by the t test.
Results Of 6029 deliveries during the study period, 103 twin pregnancies occurred. Therefore the incidence of twinning was 1.7% among viable pregnancies. Thirty-one percent of the twin gestations were monochorionic and 62.1 % were dichorionic. The remaining 6.9% (seven
Dividing membranes in twins
Volume Hi! Number 6, Part I
1541
Table I. Placental chorionicity in twin gestations Membrane thIckness (mm) (n = 32)
Twin type
Range
Monochorionic Dichorionic
1.0-2.0 1.2-4.0
SD 1.4 2.4
±0.3 ±0.7
p < 0.001. Table II. Placental chorionic status in twin gestations (n = 32) Membrane thickness
<2 mm :2:2mm TOTAL
P lacenial hIStology M onochorzonlc
9 I
10
I
Dlchonortlc
Total
2 20 22
21
II
32
For dichorionic twinning: sensitivity = 90.9%; specificity = 90.0%; positive predictive value = 95.2%; negative predictive value = 81.8%. pregnancies) consisted of twin pregnancies that did not have histologic placental examinations, These seven twin gestations consisted of five sets of the same gender and two sets of different genders. Among 103 twin pregnancies, only 32 twin pregnancies had both ultrasonographic examinations of the membranes (eight during the second trimester and 24 during the third trimester) and pathologic confirmation. The membrane thickness was essentially unchanged from the second to third trimester in four patients who had serial measurements of the dividing membranes, There was a significant difference in the mean membrane thickness between the monochorionic and dichorionic membranes (p < 0.001) (Table I). With a thickness of 2 mm used as a cutoff point, the accuracy in predicting monochorionic and dichorionic membranes was 82% and 95%, respectively (Table II), The interobserver and intraobserver variability of measurements was 15% and 10%, respectively,
Comment The overall incidence and distribution of monochorionic and dichorionic twin gestations in our patient populations were comparable to those previously reported, The incidence of twinning in the United States ranges from 1% to 2% with dichorionic placentations being present in two of three twin gestations. I 4 Determination of the type of placental chorion is helpful in the management of twin gestations. Vascular communications between the two fetal circulations occur almost exclusively in monochorionic twinning, and al-
=
2.8mm
Fig. 1. The thi('kness of the dividIng membranes was measured between the outer borders ( +). The segment chosen was perpendicular to the ultrasonography beams,
most all monochorionic placentas have some degree of vascular anastomoses. 5 . 6 Clinically significant twin-twin transfusion occurs in about 20% to 30% of monochorionic twinning. 7 Therefore monochorionic twinning requires more intense surveillance of fetal growth and well-being. Although growth discrepancy may occur in both monochorionic and dichorionic twinning, the diagnosis of twin-to-twin transfusion, which has a more omnious prognosis, should be strongly considered in monochorionic gestations. Furthermore, when fetal death occurs, the surviving twin with the monochorionic placenta may suffer thromboembolic complications resulting in multicystic encephalomalacia." disseminated intravascular coagulation, and infarction of other visceral organs." As a result, selective feticide of one twin would be ill-advised in the case of monochorionic twinning if one twin should have a congenital anomaly. In general. there is a high discordancy rate for congenital anomalies between twin pairs. III Identification of monochorionic placentas, however, indicates monozygotic twinning, which usually has identical chromosomal anomalies.
1542 Winn et al.
Histologically, the dichorionic dividing membranes are thicker than the monochorionic membranes, because the former contain two layers of chorion and two layers of amnion whereas the latter contain only two layers of amnion. Ultrasonography has been used recently for the prenatal diagnosis of placental chorion status, which is based on the difference in thickness between the two types of membranes. Qualitative evaluation of the membranes by inspection has been reported with a high degree of accuracY.3 This method, however, is applicable when the membrane thickness is at the extremes of measurement and depends, to a great extent, on the ultrasonographer's experience. We have found that there was a significant difference in mean membrane thickness between monochorionic and dichorionic placentas, and often the difference in membrane thickness is < 1 mm and may not be accurately assessed by simple inspection. Our results also demonstrated that the membranes of monochorionic gestations are measurable. In conclusion, the determination of the types of placental chorionic type is helpful in the management of twin gestations. Ultrasonography has permitted prenatal diagnosis of monochorionic and dichorionic twinning with a high degree of accuracy by identifying the number of placentas and fetal gender and measuring the membrane thickness. It should be emphasized that these measurements should be done only with high-
December 1989 Am J Obstet Gynecol
resolution ultrasonography machines and that, even with quantitation of the membrane thickness, the two types of chorionicity cannot always be distinguished. A prospective study is being conducted to confirm these findings. REFERENCES 1. Powers WF. Twin pregnancy: complications and treatment. Obstet Gynecol 1973;42:795. 2. Mahony BS, Filly RA, Callen PW. Amnionicity and chorionicity in twin pregnancies: prediction using ultrasound. Radiology 1985; 155: 205-9. 3. Barss VA, Benacerraf BR, Frigoletto FD. Ultrasonographic determination of chorion type in twin gestation. Obstet GynecoI1985;66:779-82. 4. Hertzberg BS, Kurtz AB, Choi HY, et al. Significance of membrane thickness in the sonographic evaluation of twin gestations. A]R 1987;148:151-3. 5. Benirschke K, Chung KK. Multiple pregnancy (first of two parts). N Engl] Med 1973;288: 1276-84. 6. Robertson EG, Neer K]. Placental injection studies in twin gestations. AM] OBSTET GYNECOL 1983;147:170-4. 7. Tan KL, Tan R, Tan SH, Tan AM. The twin transfusion syndrome. Clinical observations on 35 affected pairs. Clin Pediatr 1979;18:111-4. 8. Yoshioka H, Kadomoto Y, Mino M, et al. Multicystic encephalomalacia in liveborn twins with a stillborn macerated co-twin.] Pediatr 1979;95:798-800. 9. Moore CM, McAdams A], Sutherland]. Intrauterine disseminated intravascular coagulation. A syndrome of multiple pregnancy with a dead twin fetus.] Pediatr 1969; 74:523-8. 10. Benirschke K, Kim CK. Multiple pregnancy (second of two parts). N Engl] Med 1973;288:1329-36.
Key words: Twins, chorion, amnion, ultrasonography, prenatal diagnosis In the United States, the overall incidence of twinning is about one in 90 pregnancies. I Complications from twin pregnancies still contribute significantly to the overall perinatal mortality and morbidity. The incidence of morbidity is affected by the status of the placental chorion and amnion. Since monochorionic placentas almost always have some vascular anastomoses between the two fetal circulations, identification of placental chorion is useful in the management of a twin pregnancy, especially when it is complicated by growth discrepancy, fetal hydrops, or fetal death. Whereas the identification of different fetal genders or two separate placentas indicates dichorionic twinning,2 differentiation between monochorionic and dichorionic placentas must rely on other parameters when only one placenta or similar genders exist. Qualitative evaluation of the dividing membranes by ultrasonography has been attempted to determine the type of placental chorion.' The current study was undertaken, however, to quantitatively establish the so no graphic criteria for the prenatal diagnosis of chorion status in twin gestations.
Patients and methods This study was conducted at the Perinatal Unit of Yale-New Haven Hospital during the period January 1987 to January 1988. Indications for ultrasonographic From the DivislOn of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology," and the Department of Pathology,' Yale University School of Medlcl1te. Presented at the Thlrty-fifth Annual Meeting of the Society for G.vnecologie invest!gatlOn, Balt!more, Maryland, March 17-20, 1988 Reprint requests: Hung N. Winll , MD, Department of Obstetrics and Gynecology, Washington University Medical Center, 4911 Barnes Hospital Plaza, St. LOUIS, MO 63110. 6/6/162J5
1540
examination included confirmation of twin gestation, assessment of fetal growth, and genetic amniocentesis. The dividing membranes between the twins were evaluated as part of the ultrasonographic examination. After the membranes were well visualized and delineated, their thickness was measured between the outer borders, perpendicular to the long axis of the membrane segment (Fig. I), without prior knowledge of fetal gender or number of placentas. The segment of the dividing membranes used was as perpendicular to the ultrasonographic beams as possible to utilize the better axial resolution. The ultrasonographic scans were obtained with the Acuson-128 ultrasonography machine (Acuson, Mountinview, Calif) with 3.5 and 5 MHz transducers and an axial resolution of about 0.5 mm. Placental chorionic type was confirmed by histologic examination after delivery. The cutoff point of the membrane thickness measured by ultrasonography to differentiate between monochorionic and dichorionic placentas was retrospectively determined on the basis of results of the histologic examinations. Thirty-two twin pregnancies with dividing membranes evaluated both ultrasonographically and histologically formed the basis of this report. In four patients who had longitudinal ultrasonographic examinations of the dividing membranes, only one measurement for each patient was used to determine the cutoff point of membrane thickness (Tables I and II). The difference between the mean membrane thickness was analyzed by the t test.
Results Of 6029 deliveries during the study period, 103 twin pregnancies occurred. Therefore the incidence of twinning was 1.7% among viable pregnancies. Thirty-one percent of the twin gestations were monochorionic and 62.1 % were dichorionic. The remaining 6.9% (seven
Dividing membranes in twins
Volume Hi! Number 6, Part I
1541
Table I. Placental chorionicity in twin gestations Membrane thIckness (mm) (n = 32)
Twin type
Range
Monochorionic Dichorionic
1.0-2.0 1.2-4.0
SD 1.4 2.4
±0.3 ±0.7
p < 0.001. Table II. Placental chorionic status in twin gestations (n = 32) Membrane thickness
<2 mm :2:2mm TOTAL
P lacenial hIStology M onochorzonlc
9 I
10
I
Dlchonortlc
Total
2 20 22
21
II
32
For dichorionic twinning: sensitivity = 90.9%; specificity = 90.0%; positive predictive value = 95.2%; negative predictive value = 81.8%. pregnancies) consisted of twin pregnancies that did not have histologic placental examinations, These seven twin gestations consisted of five sets of the same gender and two sets of different genders. Among 103 twin pregnancies, only 32 twin pregnancies had both ultrasonographic examinations of the membranes (eight during the second trimester and 24 during the third trimester) and pathologic confirmation. The membrane thickness was essentially unchanged from the second to third trimester in four patients who had serial measurements of the dividing membranes, There was a significant difference in the mean membrane thickness between the monochorionic and dichorionic membranes (p < 0.001) (Table I). With a thickness of 2 mm used as a cutoff point, the accuracy in predicting monochorionic and dichorionic membranes was 82% and 95%, respectively (Table II), The interobserver and intraobserver variability of measurements was 15% and 10%, respectively,
Comment The overall incidence and distribution of monochorionic and dichorionic twin gestations in our patient populations were comparable to those previously reported, The incidence of twinning in the United States ranges from 1% to 2% with dichorionic placentations being present in two of three twin gestations. I 4 Determination of the type of placental chorion is helpful in the management of twin gestations. Vascular communications between the two fetal circulations occur almost exclusively in monochorionic twinning, and al-
=
2.8mm
Fig. 1. The thi('kness of the dividIng membranes was measured between the outer borders ( +). The segment chosen was perpendicular to the ultrasonography beams,
most all monochorionic placentas have some degree of vascular anastomoses. 5 . 6 Clinically significant twin-twin transfusion occurs in about 20% to 30% of monochorionic twinning. 7 Therefore monochorionic twinning requires more intense surveillance of fetal growth and well-being. Although growth discrepancy may occur in both monochorionic and dichorionic twinning, the diagnosis of twin-to-twin transfusion, which has a more omnious prognosis, should be strongly considered in monochorionic gestations. Furthermore, when fetal death occurs, the surviving twin with the monochorionic placenta may suffer thromboembolic complications resulting in multicystic encephalomalacia." disseminated intravascular coagulation, and infarction of other visceral organs." As a result, selective feticide of one twin would be ill-advised in the case of monochorionic twinning if one twin should have a congenital anomaly. In general. there is a high discordancy rate for congenital anomalies between twin pairs. III Identification of monochorionic placentas, however, indicates monozygotic twinning, which usually has identical chromosomal anomalies.
1542 Winn et al.
Histologically, the dichorionic dividing membranes are thicker than the monochorionic membranes, because the former contain two layers of chorion and two layers of amnion whereas the latter contain only two layers of amnion. Ultrasonography has been used recently for the prenatal diagnosis of placental chorion status, which is based on the difference in thickness between the two types of membranes. Qualitative evaluation of the membranes by inspection has been reported with a high degree of accuracY.3 This method, however, is applicable when the membrane thickness is at the extremes of measurement and depends, to a great extent, on the ultrasonographer's experience. We have found that there was a significant difference in mean membrane thickness between monochorionic and dichorionic placentas, and often the difference in membrane thickness is < 1 mm and may not be accurately assessed by simple inspection. Our results also demonstrated that the membranes of monochorionic gestations are measurable. In conclusion, the determination of the types of placental chorionic type is helpful in the management of twin gestations. Ultrasonography has permitted prenatal diagnosis of monochorionic and dichorionic twinning with a high degree of accuracy by identifying the number of placentas and fetal gender and measuring the membrane thickness. It should be emphasized that these measurements should be done only with high-
December 1989 Am J Obstet Gynecol
resolution ultrasonography machines and that, even with quantitation of the membrane thickness, the two types of chorionicity cannot always be distinguished. A prospective study is being conducted to confirm these findings. REFERENCES 1. Powers WF. Twin pregnancy: complications and treatment. Obstet Gynecol 1973;42:795. 2. Mahony BS, Filly RA, Callen PW. Amnionicity and chorionicity in twin pregnancies: prediction using ultrasound. Radiology 1985; 155: 205-9. 3. Barss VA, Benacerraf BR, Frigoletto FD. Ultrasonographic determination of chorion type in twin gestation. Obstet GynecoI1985;66:779-82. 4. Hertzberg BS, Kurtz AB, Choi HY, et al. Significance of membrane thickness in the sonographic evaluation of twin gestations. A]R 1987;148:151-3. 5. Benirschke K, Chung KK. Multiple pregnancy (first of two parts). N Engl] Med 1973;288: 1276-84. 6. Robertson EG, Neer K]. Placental injection studies in twin gestations. AM] OBSTET GYNECOL 1983;147:170-4. 7. Tan KL, Tan R, Tan SH, Tan AM. The twin transfusion syndrome. Clinical observations on 35 affected pairs. Clin Pediatr 1979;18:111-4. 8. Yoshioka H, Kadomoto Y, Mino M, et al. Multicystic encephalomalacia in liveborn twins with a stillborn macerated co-twin.] Pediatr 1979;95:798-800. 9. Moore CM, McAdams A], Sutherland]. Intrauterine disseminated intravascular coagulation. A syndrome of multiple pregnancy with a dead twin fetus.] Pediatr 1969; 74:523-8. 10. Benirschke K, Kim CK. Multiple pregnancy (second of two parts). N Engl] Med 1973;288:1329-36.