AMERICAN JOURNAL OF PERINATOLOGY/VOLUME 9, NUMBER 5/6
Sept/Nov 1992
IS THERE A "DOMINANT TWIN" IN UTERO? David M. Sherer, M.D., Mark N. Nawrocki, M.D., Jacques S. Abramowicz, M.D., Nancy E. Peco, R.N., Leon A. Metlay, M.D., and James R. Woods, Jr., M.D.
A prospective study of 98 pairs of nonstress tests (NSTs) was obtained simultaneously from members of 30 twin gestations. Fifty-five percent of the total fetal heart rate (FHR) accelerations of both twins were associated with each other and occurred within 15 seconds of each other. There were 8.46% synchronous FHR accelerations of both twins, possibly reflecting evoked fetal movements. Statistical analysis failed to reveal evidence of a dominant twin in utero, with one twin initiating the majority of coinciding fetal movements with statistical significance in only three gestations. Examination of combinations of the 30 separate individual significance tests using techniques of meta-analysis demonstrated a p value of 0.81, indicating lack of dominance in any of the 30 pairs of twins. The probability plot, consistent with the large p value for the combination test statistic, suggested that fetal movements in two gestations are related, strengthening the hypothesis of in utero tactile communication between twins. We conclude that the existence ofa "dominanttwin" in utero cannot be detected by electronic FHRmonitoring methods.
Previous studies have shown a 57 to 58% occurrence rate of associated fetal heart rate (FHR) accelerations in members of twin gestations during simultaneous monitoring.1-2 It has been recently postulated that the underlying mechanism of this clinical observation may be similar to that of the vibratory acoustic stimulation, whereby the second fetus responds to tactile stimulation created in utero by movements of the first twin.2 The occurrence of coinciding or associated FHR accelerations of twins raises the question: Are FHR accelerations initiated repetitively by the same twin or is this phenomenon a random event? Worded differently, does one twin dominate? The following study was designed to answer this question. MATERIALS AND METHODS During a 10-month period from November 1,1989, to August 31,1990, simultaneous nonstress tests (NSTs) of twin gestations were performed with the following technique: With the aid of a small portable real-time ultrasound unit (Aloka 210, Corometrics Medical Systems, Wallingford, CT, with a 3.0 MHz linear transducer, UST-5015,3), fetal positioning was determined. Persistent similar fetal positioning (not necessarily presentation) was a prerequisite for inclusion of each subsequent study. The location of each
fetal heart was determined with ultrasound. Two separate cardiotocograph machines (Hewlett Packard, Waltham, MA, 8040A and 8041 A) were utilized. The separate transducers were positioned above each respective fetal heart. The paper speed was 3 cm/min. For future reference and to assure that both fetuses were being monitored, each printout was marked either at the beginning or end of each test and the pairs of FHRs were noted to be different. In the latter part of the study, a single cardiotocograph machine capable of performing simultaneous external FHR monitoring of twins (Hewlett Packard, 1350A), was employed. The tests were performed by one of three senior registered nurses every 2 weeks on all of the included cases as part of antepartum surveillance of twin gestations. All nonlaboring patients with viable twin gestations were included in the study, irrespective of fetal lie, presentation, maternal disease, or obstetric complications. Patients with uterine contractions, premature rupture of the membranes, or major fetal anomalies were omitted from the study (n = 3). Cases were excluded for the following reason: nonreactive NSTs or reactive minutes of NSTs following cases in which external fetal vibroacoustic stimulation (n = 2), and persistent fetal positioning could not be confirmed (n = 2). All attempts at obtaining simultaneous monitoring were successful. FHR accelerations were defined as accelerations of at least 15 beats/min above base-
Division of Maternal-Fetal Medicine, The Department of Obstetrics and Gynecology, Strong Memorial Hospital, The University of Rochester School of Medicine and Dentistry, Rochester, New York Reprint requests: Dr. Sherer, Department of Obstetrics and Gynecology, Division of Maternal-Fetal Medicine, Strong Memorial Hospital, The University of Rochester School of Medicine and Dentistry, 601 Elmwood Avenue, Box 668, Rochester, NY 14642 460
Copyright © 1992 by Thieme Medical Publishers, Inc., 381 Park Avenue South, New York, NY 10016. All rights reserved.
Downloaded by: Scott Memorial Library, AISR. Copyrighted material.
ABSTRACT
DOMINANT TWIN IN UTERO/Sherer, et al.
line for longer than 15 seconds.3 FHR accelerations were noted to be associated only if the accelerations of each respective twin occurred within 15 seconds after the onset of the other. An FHR acceleration occurring 15 seconds or less before that of the cotwin's associated FHR acceleration was identified as the initiating FHR acceleration (Figs. 1,2). Synchronous FHR accelerations were not attributed to either twin. Brief FHR acceleration of one twin and a continuous prolonged acceleration of the other twin occurring together, although coinciding, were regarded as coincidental and not as associated FHR accelerations. Strict criteria were applied in interpreting the monitoring recordings. All NSTs were interpreted independently by each of two authors (DMS, MNN) and FHR accelerations were then reread and checked by the other author and points of
- 1 —
RESULTS
Thirty twin gestations, with a total of 98 simultaneous pairs of NSTs, were evaluated. A mean of three NSTs were performed for each twin gestation (range, one to seven). The perinatal data were as follows: Mean maternal age 30
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Downloaded by: Scott Memorial Library, AISR. Copyrighted material.
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Sept/Nov 1992
IS THERE A "DOMINANT TWIN" IN UTERO? David M. Sherer, M.D., Mark N. Nawrocki, M.D., Jacques S. Abramowicz, M.D., Nancy E. Peco, R.N., Leon A. Metlay, M.D., and James R. Woods, Jr., M.D.
A prospective study of 98 pairs of nonstress tests (NSTs) was obtained simultaneously from members of 30 twin gestations. Fifty-five percent of the total fetal heart rate (FHR) accelerations of both twins were associated with each other and occurred within 15 seconds of each other. There were 8.46% synchronous FHR accelerations of both twins, possibly reflecting evoked fetal movements. Statistical analysis failed to reveal evidence of a dominant twin in utero, with one twin initiating the majority of coinciding fetal movements with statistical significance in only three gestations. Examination of combinations of the 30 separate individual significance tests using techniques of meta-analysis demonstrated a p value of 0.81, indicating lack of dominance in any of the 30 pairs of twins. The probability plot, consistent with the large p value for the combination test statistic, suggested that fetal movements in two gestations are related, strengthening the hypothesis of in utero tactile communication between twins. We conclude that the existence ofa "dominanttwin" in utero cannot be detected by electronic FHRmonitoring methods.
Previous studies have shown a 57 to 58% occurrence rate of associated fetal heart rate (FHR) accelerations in members of twin gestations during simultaneous monitoring.1-2 It has been recently postulated that the underlying mechanism of this clinical observation may be similar to that of the vibratory acoustic stimulation, whereby the second fetus responds to tactile stimulation created in utero by movements of the first twin.2 The occurrence of coinciding or associated FHR accelerations of twins raises the question: Are FHR accelerations initiated repetitively by the same twin or is this phenomenon a random event? Worded differently, does one twin dominate? The following study was designed to answer this question. MATERIALS AND METHODS During a 10-month period from November 1,1989, to August 31,1990, simultaneous nonstress tests (NSTs) of twin gestations were performed with the following technique: With the aid of a small portable real-time ultrasound unit (Aloka 210, Corometrics Medical Systems, Wallingford, CT, with a 3.0 MHz linear transducer, UST-5015,3), fetal positioning was determined. Persistent similar fetal positioning (not necessarily presentation) was a prerequisite for inclusion of each subsequent study. The location of each
fetal heart was determined with ultrasound. Two separate cardiotocograph machines (Hewlett Packard, Waltham, MA, 8040A and 8041 A) were utilized. The separate transducers were positioned above each respective fetal heart. The paper speed was 3 cm/min. For future reference and to assure that both fetuses were being monitored, each printout was marked either at the beginning or end of each test and the pairs of FHRs were noted to be different. In the latter part of the study, a single cardiotocograph machine capable of performing simultaneous external FHR monitoring of twins (Hewlett Packard, 1350A), was employed. The tests were performed by one of three senior registered nurses every 2 weeks on all of the included cases as part of antepartum surveillance of twin gestations. All nonlaboring patients with viable twin gestations were included in the study, irrespective of fetal lie, presentation, maternal disease, or obstetric complications. Patients with uterine contractions, premature rupture of the membranes, or major fetal anomalies were omitted from the study (n = 3). Cases were excluded for the following reason: nonreactive NSTs or reactive minutes of NSTs following cases in which external fetal vibroacoustic stimulation (n = 2), and persistent fetal positioning could not be confirmed (n = 2). All attempts at obtaining simultaneous monitoring were successful. FHR accelerations were defined as accelerations of at least 15 beats/min above base-
Division of Maternal-Fetal Medicine, The Department of Obstetrics and Gynecology, Strong Memorial Hospital, The University of Rochester School of Medicine and Dentistry, Rochester, New York Reprint requests: Dr. Sherer, Department of Obstetrics and Gynecology, Division of Maternal-Fetal Medicine, Strong Memorial Hospital, The University of Rochester School of Medicine and Dentistry, 601 Elmwood Avenue, Box 668, Rochester, NY 14642 460
Copyright © 1992 by Thieme Medical Publishers, Inc., 381 Park Avenue South, New York, NY 10016. All rights reserved.
Downloaded by: Scott Memorial Library, AISR. Copyrighted material.
ABSTRACT
DOMINANT TWIN IN UTERO/Sherer, et al.
line for longer than 15 seconds.3 FHR accelerations were noted to be associated only if the accelerations of each respective twin occurred within 15 seconds after the onset of the other. An FHR acceleration occurring 15 seconds or less before that of the cotwin's associated FHR acceleration was identified as the initiating FHR acceleration (Figs. 1,2). Synchronous FHR accelerations were not attributed to either twin. Brief FHR acceleration of one twin and a continuous prolonged acceleration of the other twin occurring together, although coinciding, were regarded as coincidental and not as associated FHR accelerations. Strict criteria were applied in interpreting the monitoring recordings. All NSTs were interpreted independently by each of two authors (DMS, MNN) and FHR accelerations were then reread and checked by the other author and points of
- 1 —
RESULTS
Thirty twin gestations, with a total of 98 simultaneous pairs of NSTs, were evaluated. A mean of three NSTs were performed for each twin gestation (range, one to seven). The perinatal data were as follows: Mean maternal age 30
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Downloaded by: Scott Memorial Library, AISR. Copyrighted material.
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