American Journal of Medical Genetics 37:558-561 (1990)
Monochorionic Diamniotic Minimally Conjoined Twins: A Case Report P.J. Weston, E.J. Ives, R.L.H. Honore, G.M. Lees, D.B. Sinclair, and D. Schiff Department of Pediatrics (D.S.), Division of Newborn Medicine (P.J.W.), Division of Medical Genetics (E.J.I.), Division of Pediatric Neurology (D.B.S.), Department of Pathology, (R.L.H.H.), and Department of Surgery (G.M.L.), University of Alberta, Edmonton, Alberta, Canada
We present the second case of monochorionic diamniotic (MC/DA) conjoined twins. There was minimal conjunction, which was predominantly extrafetal and confined to the periumbilical ventral region. The omphalopagus twins, attached to a single forked umbilical cord, were connected by a shared umbilical hernia containing the ileum of twin B. The only visceral conjunction, located just within the belly of twin A, was midileal with the 2 separate ileums converging toward a short segment of shared muscularis propria and of side-to-sidefistulization. Gastrointestinal and musculoskeletal anomalies were present in both twins with severe amyoplasia and arthrogryposis multiplex in twin A. Possible mechanisms underlying this unusual form of MZ twinning are discussed. KEY WORDS: conjoined twins, omphalopagus, monochorionic, diamniotic, arthrogryposis, amyoplasia INTRODUCTION Conjoined twins constitute a rare but serious complication of monozygotic twinning; they result from the incomplete fission of the germinal disc about day 13-15 postconception and are typically monochorionic and monoamniotic [Benirschke, 1975; Corner, 1955; Zimmermann, 19671. We present the second case of pathologically documented monochorionzc diamniotic (MC/ DA) conjoined twins [Zimmermann, 19671. CLINICAL REPORT A 28-year-old Caucasian primipara had a normal pregnancy complicated by slight vaginal bleeding a t 3 Received for publication January 2, 1990; revision received March 6, 1990. Address reprint requests t o David Schiff, M.D., Ph.D., 2C3 Walter Mackenzie Health Sciences Centre, Department of Pediatrics, University of Alberta Hospitals, Edmonton, Alberta, T6G 2R7 Canada.
0 1990 Wiley-Liss, Inc.
months, a urinary tract infection a t 4 months, and a throat infection a t 5 months. Twins were diagnosed a t 33 weeks by ultrasound and a t 36 weeks she presented with a n unstable lie and cervical dilatation. Ultrasonography showed 2 fetuses in the breech position without mirror imaging, with one twin having a smaller abdomen; there was a single placenta and a n incompletely visualized dividing membrane. The diagnosis of conjoined twins was not made. Amniocentesis showed a mature L:S ratio and meconium-stained liquor. At cesarean section twin A (1,940 g male) could not be delivered because of a short thick cord, which was cut a t some distance from the body on the suspicion of an omphalocele. Twin B (2,640 g male) was then delivered with the cord and placenta, and severe hemorrhage from a cut umbilical vessel. Both twins required emergency surgery for control of hemorrhage, separation of the conjoined ileum, and repair of their abdominal wall. The severed ileum of twin B was found to lie in the umbilical hernia from where it entered the abdomen of twin A to achieve a Y-shaped conjunction just proximal to the Meckels diverticulum of twin A. There was a side-to-side fistula with the blind gut of twin B decompressing into the ileum of twin A; proximally there was a short segment of gut with two separate lumina surrounded by single muscularis propria. Twin A had a nonrotated nonfixed, otherwise normal bowel with a rectovesical fistula and a n imperforate anus; a colostomy was done. Twin B had no bowel beyond the point of conjunction and a n imperforate anus; an ileostomy was done. Postoperatively they did well and were discharged a t day 33 (Fig. 1). Twin A had rounded shoulders, flexion contractures of elbows and knees, with related pterygia and joint dimples, underdeveloped limb muscles, rigid talipes equinovarus, and narrow hands with flexion contractures of all digits. There were no capillary hemangiomata. Both twins had normal brain CT scans. Therefore, twin A was thought to have the amyoplasia variety of arthrogryposis multiplex congenita. In addition he had a patent urachus and right cryptorchidism, whereas twin B had glandular hypospadias. Both twins had a normal head circumference (50th centile). Both twins had extensive musculoskeletal studies, including radiography, muscle and nerve biopsy, CPK measurements, electromyography, ultra-
Monochorionic Diamniotic Conjoined Twins
559
Fig. 1. Schematic diagram demonstrating the umbilical hernia and confunction of the small bowel of twin B into that of twin A
sound, and CT scans of the brain. Twin A was diagnosed as having amyoplasia-related arthrogryposis multiplex congenita with plagiocephaly, hypoplasia, and diastasis of the pubic symphysis, dysplasia of the acetabula, patent urachus, and right cryptorchidism. Twin B has plagiocephaly, a dysplastic left acetabulum, small descended testes and mild glandular hypospadias (Figs. 2, 3). The meconium-stained chorionic plate of the single placenta was divided eccentrically in a ratio of 60140 in favor of twin B by an incomplete ragged membrane, spanning the entire chorionic plate and forming a slinglike investment around the single cord (Fig. 4). Sections of this membrane showed two back-to-back amnions without intervening chorion, i.e., MC1DA placentation. Only one yolk sac was found, located peripherally in the dividing membrane. The distally frayed eccentrically inserted cord was unusually short and thick (9 cm long and 5-8 cm wide) and contained 2 separate sets of blood vessels (3 to twin B + 2 to twin A) with a shallow intervening groove. Monozygosity was confirmed using hMF#1 and 33.15 min satellite probes on peripheral lymphocytes and tissue fibroblasts.
DISCUSSION These conjoined twins are unique because of, i.e., MC/ DA placentation, minimal predominantly extrafetal conjunction, and discordant amyoplasia in twin A. The presence of a n MCiDA placenta is unusual but not unique. The first case was published in 1967 with only scant details about the fetuses and no attempt a t explaining these “unusual findings” [Zimmermann, 19671. To explain this phenomenon we postulate the following sequence of events: 1. Division of the inner cell mass after the formation of the primitive chorion but before amniogenesis, i.e., about day 6-7 of development. 2. Inadequate spatial separation of the inner cell masses to prevent overlap of tissue domains. 3. Critical approximation of the abembryonic (distal) portions of the cranial halves of the secondary yolk sacs with overlap of the splanchnic mesoderms. This caused segmental back-to-back apposition of the two separate yolk sacs, fusion of their mesoderms and persistent conjunction of their derivatives, i.e., the
560
Weston et al.
Fig. 2. Anterior view of twin A demonstrating arthrogryposis involving all limbs, fusiform fingers and narrow hands, and clinical features of amyoplasia.
midileums. The mesodermal fusion led to sharing of the muscularis propria externally while internally the 2 ileal lumina stayed separate, except for the side-to-side fistula, which probably resulted from secondary rupture of the mucosal party wall, a s the intraluminal pressure increased within the blind gut of twin B, i.e., a decompressive event. 4. Formation of 2 separate germinal discs with independent amnions, mesoderms and endoderms. This ensured the formation of 2 separate bodies linked only a t the level of the originally extraembryonic cranial limb of the midgut proximal to the transient vitelline duct, persisting as Meckels diverticulum in twin A. 5. Incomplete return of the midgut into the abdomen of twin A followed by the attached ileum of twin B, which thus could not return into its own abdomen and remained in the persistent umbilical hernia on its way to the belly of twin A. 6. The extracorporeal conjunction of the 2 midileums caused approximation and fusion of the 2 body stalks into a single forked cord with persistent umbilical hernia. This speculative interpretation is based on the concept of a continuum of morphologic expression of MZ twinning, critically dependent on 2 factors, i.e., the tim-
Fig. 3. Postoperative anterior view of twin B with mild talipes equinovarus, plagiocephaly, and mid-shaft hypospadias.
Fig. 4. Fetal aspect of the placenta: chorionic plate, single umbilical cord, and sling-like diamniotic septum (arrow) between free membranes and cord.
ing fission and the degree of spatial separation of the twin primordia [Benirschke, 1975; Corner, 1955; Zimmermann, 19671. In early development, when twinning occurs, there is no sharp demarcation between embryonic (germinal disc) and extraembryonic (amnion, yolk sac, and body stalk) components and incomplete fission can involve both components or either singly. Most cases of conjoined twins occur between day 13-15 after the formation of chorion, amnion, and yolk sac, resulting in MClMA placentation and sharing of the 3 derivatives of the germinal disc (ectoderm, mesoderm, and endoderm) and of the yolk sac and body stalk. There are examples of selective involvement of these components, i.e., conjoined twins with separate midguts and cords, i.e., germinal disc conjunction [Guttmacher and Nichols, 19671 and separate monoamniotic twins with single yolk sac [Arey, 19221 or single forked umbilical cord [Larson et al., 19591, i.e., extraembryonic conjunction. Our case represents a n intermediate stage of sharing between the single cord and the omphalopagus, which tends to show complete luminal and mural sharing of a midgut segment [Gans et al., 19681. The presence of a rectovesical fistula with imperforate anus in twin A and the complete absence of distal ileum and large bowel with imperforate anus in twin B deserve comment. There is a strong association between omphalopaguslthoracoomphalopagus and hindgut defects with rectovesical fistula and imperforate anus [Edmonds and Layde, 1982; Ornoy et al., 19801. This original pattern persisted in twin A but was modified in twin B by secondary loss of the terminal ileum and large bowel, due to progressive ischemia caused by their extra abdominal location and the increasing pull exerted by
Monochorionic Diamniotic Conjoined Twins
the returning small gut of twin A. This suggestion, in keeping with the vasculogenic theory of intestinal atresia [Van Allen, 19811, is indirectly supported by the presence of a mesentery running along the left abdominal wall to the dome of the bladder. The cause of the primary hindgut defect is unknown but may well be related to the twinning process [Schinzel e t al., 19791. This is the first case of discordant amyoplasia and congenital arthrogryposis reported in monozygotic conjoined twins LHall et al., Pt 11, 1983133.The etiology and pathogenesis of amyoplasia are poorly understood but several environmental facts have been suggested as related, i.e., viral infections, vascular insults, hyperthermia, intrauterine compression and exposure to toxic chemicals such as alcohol, alkaloids, etc. [Hall et al., 1983al. In our case it is not clear why twin A was selectively afflicted by amyoplasia. It is possible that the asymmetry of the twinning process itself may predispose to discordant amyoplasia or that the normal twin may support the amyoplastic twin, which otherwise would have been aborted [Hall et al., 1983al. This case stresses the need for the prenatal diagnosis of conjoined twins [Filler, 19861 to avoid potentially lethal perinatal complications. The lesson is that the presence on ultrasound of a dividing membrane does not absolutely rule out conjoined twins and may be associated with minimal ventral conjunction, particularly difficult to recognize ultrasonically. Thus, any set of monochorionic twins, whether monoamniotic or diamniotic, deserves careful ultrasonic evaluation [Koontz et al., 19831 to exclude conjunction.
ACKNOWLEDGMENTS We would like to thank Dr. David Hoare for assessing monozygosity, Drs. S. Bamforth and G. Machin for editorial comments to the manuscript, and Joanne Pocock for secretarial assistance.
561
REFERENCES Arey LB (1922):Direct proof of the monozygotic origin of human identical twins. Anat Rec 23:245-251. Benirschke K (19751: Origin and clinical significance of twinning. Clinics Obstet Gynecol 15:220-235. Corner GW (1955):The observed embryology of human single-ovum twins and other multiple births. Am J Obstet Gynecol 70:933-951. Edmonds LD, Layde PM (1982):Conjoined twins in the United States, 1970-1977. Teratology 25:301-308. Filler RM (1986): Conjoined twins and their separation. Semin Perinatol 10:82-91. Gans SL, Morgenstern L, Gettelmane E, Zukow AH, Cooperman H (1968): Separation of conjoined twins in the newborn period. J Pediatr Surg 3:565-574. Guttmacher AF, Nichols BL (1967): Conjoined Twins. In Bergsma D (ed):“Teratology of Conjoined Twins.” New York: Alan R. Liss, Inc., for the National Foundation-March of Dimes, BD:OAS 111(1):3-9. Hall JG, Reed SD, Driscoll EP (1983ai: Part I. Amyoplasia: A common, sporadic condition with congenital contractures. Am J Med Genet 15:571-590. Hall JG, Reed SD, Driscoll EP (198313):Part 11. Amyoplasia: Twinning in amyoplasia-a specific type of arthrogryposis with an apparent excess of discordantly affected identical twins. Am J Med Genet 15:591-599. Koontz WL, Herbert WNP, Seeds JW, Cefalo RC (1983):Ultrasonography in the antepartum diagnosis of conjoined twins. J Reprod Med 28:627-632. Larson SL, Kempers RD, Titus J L (1959):Monoamniotic twins with a common umbilical cord. Am J Obstet Gynecol 105:635-636. Ornoy A, Navot D, Menashi M, Laufer N, Chemke J (1980):Asymmetry and discordance for congenital anomalies in conjoined twins: A report of six cases. Teratology 22:145-164. Schinzel A, Smith DW, Miller J R (1979): Monozygotic twinning and structural defects. J Pediatr 95:921-930. Van Allen MI (1981):Fetal vascular disruptions: Mechanisms and some resulting birth defects. Ped Ann 10:219-233. Wiegenstein L, Iozzo RV (1980): Unusual findings in a conjoined (“Siamese”) twin placenta. Am J Obstet Gynecol 137:744-745. Zimmermann AA (1967):Conjoined twins. In Bergsma D (ed):“Embryologic and Anatomic Considerations of Conjoined Twins.” New York: Alan R. Liss, Inc., for the National Foundation-March of Dimes, BD:OAS III(1):18-27.
Monochorionic Diamniotic Minimally Conjoined Twins: A Case Report P.J. Weston, E.J. Ives, R.L.H. Honore, G.M. Lees, D.B. Sinclair, and D. Schiff Department of Pediatrics (D.S.), Division of Newborn Medicine (P.J.W.), Division of Medical Genetics (E.J.I.), Division of Pediatric Neurology (D.B.S.), Department of Pathology, (R.L.H.H.), and Department of Surgery (G.M.L.), University of Alberta, Edmonton, Alberta, Canada
We present the second case of monochorionic diamniotic (MC/DA) conjoined twins. There was minimal conjunction, which was predominantly extrafetal and confined to the periumbilical ventral region. The omphalopagus twins, attached to a single forked umbilical cord, were connected by a shared umbilical hernia containing the ileum of twin B. The only visceral conjunction, located just within the belly of twin A, was midileal with the 2 separate ileums converging toward a short segment of shared muscularis propria and of side-to-sidefistulization. Gastrointestinal and musculoskeletal anomalies were present in both twins with severe amyoplasia and arthrogryposis multiplex in twin A. Possible mechanisms underlying this unusual form of MZ twinning are discussed. KEY WORDS: conjoined twins, omphalopagus, monochorionic, diamniotic, arthrogryposis, amyoplasia INTRODUCTION Conjoined twins constitute a rare but serious complication of monozygotic twinning; they result from the incomplete fission of the germinal disc about day 13-15 postconception and are typically monochorionic and monoamniotic [Benirschke, 1975; Corner, 1955; Zimmermann, 19671. We present the second case of pathologically documented monochorionzc diamniotic (MC/ DA) conjoined twins [Zimmermann, 19671. CLINICAL REPORT A 28-year-old Caucasian primipara had a normal pregnancy complicated by slight vaginal bleeding a t 3 Received for publication January 2, 1990; revision received March 6, 1990. Address reprint requests t o David Schiff, M.D., Ph.D., 2C3 Walter Mackenzie Health Sciences Centre, Department of Pediatrics, University of Alberta Hospitals, Edmonton, Alberta, T6G 2R7 Canada.
0 1990 Wiley-Liss, Inc.
months, a urinary tract infection a t 4 months, and a throat infection a t 5 months. Twins were diagnosed a t 33 weeks by ultrasound and a t 36 weeks she presented with a n unstable lie and cervical dilatation. Ultrasonography showed 2 fetuses in the breech position without mirror imaging, with one twin having a smaller abdomen; there was a single placenta and a n incompletely visualized dividing membrane. The diagnosis of conjoined twins was not made. Amniocentesis showed a mature L:S ratio and meconium-stained liquor. At cesarean section twin A (1,940 g male) could not be delivered because of a short thick cord, which was cut a t some distance from the body on the suspicion of an omphalocele. Twin B (2,640 g male) was then delivered with the cord and placenta, and severe hemorrhage from a cut umbilical vessel. Both twins required emergency surgery for control of hemorrhage, separation of the conjoined ileum, and repair of their abdominal wall. The severed ileum of twin B was found to lie in the umbilical hernia from where it entered the abdomen of twin A to achieve a Y-shaped conjunction just proximal to the Meckels diverticulum of twin A. There was a side-to-side fistula with the blind gut of twin B decompressing into the ileum of twin A; proximally there was a short segment of gut with two separate lumina surrounded by single muscularis propria. Twin A had a nonrotated nonfixed, otherwise normal bowel with a rectovesical fistula and a n imperforate anus; a colostomy was done. Twin B had no bowel beyond the point of conjunction and a n imperforate anus; an ileostomy was done. Postoperatively they did well and were discharged a t day 33 (Fig. 1). Twin A had rounded shoulders, flexion contractures of elbows and knees, with related pterygia and joint dimples, underdeveloped limb muscles, rigid talipes equinovarus, and narrow hands with flexion contractures of all digits. There were no capillary hemangiomata. Both twins had normal brain CT scans. Therefore, twin A was thought to have the amyoplasia variety of arthrogryposis multiplex congenita. In addition he had a patent urachus and right cryptorchidism, whereas twin B had glandular hypospadias. Both twins had a normal head circumference (50th centile). Both twins had extensive musculoskeletal studies, including radiography, muscle and nerve biopsy, CPK measurements, electromyography, ultra-
Monochorionic Diamniotic Conjoined Twins
559
Fig. 1. Schematic diagram demonstrating the umbilical hernia and confunction of the small bowel of twin B into that of twin A
sound, and CT scans of the brain. Twin A was diagnosed as having amyoplasia-related arthrogryposis multiplex congenita with plagiocephaly, hypoplasia, and diastasis of the pubic symphysis, dysplasia of the acetabula, patent urachus, and right cryptorchidism. Twin B has plagiocephaly, a dysplastic left acetabulum, small descended testes and mild glandular hypospadias (Figs. 2, 3). The meconium-stained chorionic plate of the single placenta was divided eccentrically in a ratio of 60140 in favor of twin B by an incomplete ragged membrane, spanning the entire chorionic plate and forming a slinglike investment around the single cord (Fig. 4). Sections of this membrane showed two back-to-back amnions without intervening chorion, i.e., MC1DA placentation. Only one yolk sac was found, located peripherally in the dividing membrane. The distally frayed eccentrically inserted cord was unusually short and thick (9 cm long and 5-8 cm wide) and contained 2 separate sets of blood vessels (3 to twin B + 2 to twin A) with a shallow intervening groove. Monozygosity was confirmed using hMF#1 and 33.15 min satellite probes on peripheral lymphocytes and tissue fibroblasts.
DISCUSSION These conjoined twins are unique because of, i.e., MC/ DA placentation, minimal predominantly extrafetal conjunction, and discordant amyoplasia in twin A. The presence of a n MCiDA placenta is unusual but not unique. The first case was published in 1967 with only scant details about the fetuses and no attempt a t explaining these “unusual findings” [Zimmermann, 19671. To explain this phenomenon we postulate the following sequence of events: 1. Division of the inner cell mass after the formation of the primitive chorion but before amniogenesis, i.e., about day 6-7 of development. 2. Inadequate spatial separation of the inner cell masses to prevent overlap of tissue domains. 3. Critical approximation of the abembryonic (distal) portions of the cranial halves of the secondary yolk sacs with overlap of the splanchnic mesoderms. This caused segmental back-to-back apposition of the two separate yolk sacs, fusion of their mesoderms and persistent conjunction of their derivatives, i.e., the
560
Weston et al.
Fig. 2. Anterior view of twin A demonstrating arthrogryposis involving all limbs, fusiform fingers and narrow hands, and clinical features of amyoplasia.
midileums. The mesodermal fusion led to sharing of the muscularis propria externally while internally the 2 ileal lumina stayed separate, except for the side-to-side fistula, which probably resulted from secondary rupture of the mucosal party wall, a s the intraluminal pressure increased within the blind gut of twin B, i.e., a decompressive event. 4. Formation of 2 separate germinal discs with independent amnions, mesoderms and endoderms. This ensured the formation of 2 separate bodies linked only a t the level of the originally extraembryonic cranial limb of the midgut proximal to the transient vitelline duct, persisting as Meckels diverticulum in twin A. 5. Incomplete return of the midgut into the abdomen of twin A followed by the attached ileum of twin B, which thus could not return into its own abdomen and remained in the persistent umbilical hernia on its way to the belly of twin A. 6. The extracorporeal conjunction of the 2 midileums caused approximation and fusion of the 2 body stalks into a single forked cord with persistent umbilical hernia. This speculative interpretation is based on the concept of a continuum of morphologic expression of MZ twinning, critically dependent on 2 factors, i.e., the tim-
Fig. 3. Postoperative anterior view of twin B with mild talipes equinovarus, plagiocephaly, and mid-shaft hypospadias.
Fig. 4. Fetal aspect of the placenta: chorionic plate, single umbilical cord, and sling-like diamniotic septum (arrow) between free membranes and cord.
ing fission and the degree of spatial separation of the twin primordia [Benirschke, 1975; Corner, 1955; Zimmermann, 19671. In early development, when twinning occurs, there is no sharp demarcation between embryonic (germinal disc) and extraembryonic (amnion, yolk sac, and body stalk) components and incomplete fission can involve both components or either singly. Most cases of conjoined twins occur between day 13-15 after the formation of chorion, amnion, and yolk sac, resulting in MClMA placentation and sharing of the 3 derivatives of the germinal disc (ectoderm, mesoderm, and endoderm) and of the yolk sac and body stalk. There are examples of selective involvement of these components, i.e., conjoined twins with separate midguts and cords, i.e., germinal disc conjunction [Guttmacher and Nichols, 19671 and separate monoamniotic twins with single yolk sac [Arey, 19221 or single forked umbilical cord [Larson et al., 19591, i.e., extraembryonic conjunction. Our case represents a n intermediate stage of sharing between the single cord and the omphalopagus, which tends to show complete luminal and mural sharing of a midgut segment [Gans et al., 19681. The presence of a rectovesical fistula with imperforate anus in twin A and the complete absence of distal ileum and large bowel with imperforate anus in twin B deserve comment. There is a strong association between omphalopaguslthoracoomphalopagus and hindgut defects with rectovesical fistula and imperforate anus [Edmonds and Layde, 1982; Ornoy et al., 19801. This original pattern persisted in twin A but was modified in twin B by secondary loss of the terminal ileum and large bowel, due to progressive ischemia caused by their extra abdominal location and the increasing pull exerted by
Monochorionic Diamniotic Conjoined Twins
the returning small gut of twin A. This suggestion, in keeping with the vasculogenic theory of intestinal atresia [Van Allen, 19811, is indirectly supported by the presence of a mesentery running along the left abdominal wall to the dome of the bladder. The cause of the primary hindgut defect is unknown but may well be related to the twinning process [Schinzel e t al., 19791. This is the first case of discordant amyoplasia and congenital arthrogryposis reported in monozygotic conjoined twins LHall et al., Pt 11, 1983133.The etiology and pathogenesis of amyoplasia are poorly understood but several environmental facts have been suggested as related, i.e., viral infections, vascular insults, hyperthermia, intrauterine compression and exposure to toxic chemicals such as alcohol, alkaloids, etc. [Hall et al., 1983al. In our case it is not clear why twin A was selectively afflicted by amyoplasia. It is possible that the asymmetry of the twinning process itself may predispose to discordant amyoplasia or that the normal twin may support the amyoplastic twin, which otherwise would have been aborted [Hall et al., 1983al. This case stresses the need for the prenatal diagnosis of conjoined twins [Filler, 19861 to avoid potentially lethal perinatal complications. The lesson is that the presence on ultrasound of a dividing membrane does not absolutely rule out conjoined twins and may be associated with minimal ventral conjunction, particularly difficult to recognize ultrasonically. Thus, any set of monochorionic twins, whether monoamniotic or diamniotic, deserves careful ultrasonic evaluation [Koontz et al., 19831 to exclude conjunction.
ACKNOWLEDGMENTS We would like to thank Dr. David Hoare for assessing monozygosity, Drs. S. Bamforth and G. Machin for editorial comments to the manuscript, and Joanne Pocock for secretarial assistance.
561
REFERENCES Arey LB (1922):Direct proof of the monozygotic origin of human identical twins. Anat Rec 23:245-251. Benirschke K (19751: Origin and clinical significance of twinning. Clinics Obstet Gynecol 15:220-235. Corner GW (1955):The observed embryology of human single-ovum twins and other multiple births. Am J Obstet Gynecol 70:933-951. Edmonds LD, Layde PM (1982):Conjoined twins in the United States, 1970-1977. Teratology 25:301-308. Filler RM (1986): Conjoined twins and their separation. Semin Perinatol 10:82-91. Gans SL, Morgenstern L, Gettelmane E, Zukow AH, Cooperman H (1968): Separation of conjoined twins in the newborn period. J Pediatr Surg 3:565-574. Guttmacher AF, Nichols BL (1967): Conjoined Twins. In Bergsma D (ed):“Teratology of Conjoined Twins.” New York: Alan R. Liss, Inc., for the National Foundation-March of Dimes, BD:OAS 111(1):3-9. Hall JG, Reed SD, Driscoll EP (1983ai: Part I. Amyoplasia: A common, sporadic condition with congenital contractures. Am J Med Genet 15:571-590. Hall JG, Reed SD, Driscoll EP (198313):Part 11. Amyoplasia: Twinning in amyoplasia-a specific type of arthrogryposis with an apparent excess of discordantly affected identical twins. Am J Med Genet 15:591-599. Koontz WL, Herbert WNP, Seeds JW, Cefalo RC (1983):Ultrasonography in the antepartum diagnosis of conjoined twins. J Reprod Med 28:627-632. Larson SL, Kempers RD, Titus J L (1959):Monoamniotic twins with a common umbilical cord. Am J Obstet Gynecol 105:635-636. Ornoy A, Navot D, Menashi M, Laufer N, Chemke J (1980):Asymmetry and discordance for congenital anomalies in conjoined twins: A report of six cases. Teratology 22:145-164. Schinzel A, Smith DW, Miller J R (1979): Monozygotic twinning and structural defects. J Pediatr 95:921-930. Van Allen MI (1981):Fetal vascular disruptions: Mechanisms and some resulting birth defects. Ped Ann 10:219-233. Wiegenstein L, Iozzo RV (1980): Unusual findings in a conjoined (“Siamese”) twin placenta. Am J Obstet Gynecol 137:744-745. Zimmermann AA (1967):Conjoined twins. In Bergsma D (ed):“Embryologic and Anatomic Considerations of Conjoined Twins.” New York: Alan R. Liss, Inc., for the National Foundation-March of Dimes, BD:OAS III(1):18-27.
