FIGURE 45-14 Types of conjoined twins. (Modified with permission from Spencer R: Theoretical and analytical embryology of conjoined . Parapagus Parapagus twins: part I: embryogenesis, (lin Anat. 2000;13(1 ):36-53.) series in Seminars in Pediatric Surgey with a preface by Spitz (2015) provide an excellent reference regarding postnatal management.
Viable conjoined twins should be delivered by cesarean. For the purpose of pregnancy termination, however, vaginal delivery is possible because the union is most often pliable (Fig. 45-17). Still, dystocia is common, and if the fetuses are mature, vaginal delivery may be traumatic to the uterus or cervix.
his is a grossly defective fetus or merely fetal parts, attached externally to a relatively normal twin. A parasitic twin usually consists of externally attached supernumerary limbs, often with some viscera. Classically, however, a functional heart or brain is absent. Attachment mirrors those sites described earlier for conjoined twins (see Fig. 45-14). Parasites are believed to result from demise of the defective twin. Its surviving tissue attaches to and receives vascularity from the normal cotwin (Spencer, 2001). In one large epidemiological study, parasitic twins accounted for 4 percent of all conjoined twins and occurred more frequently in male fetuses (Mutchinick, 2011).
FIGURE 45-15 Sonogram of a conjoined twin pregnancy at 13 weeks' gestation. These thoracoomphalopagus twins have two heads but a shared chest and abdomen.
Early in development, one embryo may be enfolded within its twin. Normal development of this rare parasitic twin usually arrests in the first trimester. As a result, norml spatial arrangement of and presence of many organs is lost. Classically, vertebral or xial bones are found in the fetiform mass, whereas a heart and brain are absent. hese masses are believed to represent a monozygotic, monochorionic diamnionic twin gestation and are typically supported by large parasitic vessels to the host (McNamara, 2016; Spencer, 2000). Malignant degeneration is rare (Kaufman, 2007).
All monochorionic placentas likely share some anastomotic connections. And, with rare exceptions, anastomoses between
FIGURE 45-16 Magnetic resonance imaging of conjoined twins. This T2-weighted HASTE sagittal image demonstrates fusion from the level of the xiphoid process to just below the level of the umbilicus, that is, omphalopagus twins. Below the fused liver (L)/ there is a midline cystic mass (arrow) within the tissue connecting the twins. An omphalomesenteric cyst was favored given the location within the shared tissue. (Used with permission from
Dr. April Bailey.)
FIGURE 45-17 Conjoined twins aborted at 17 weeks' gestation. (Used with permission from Dr. Jonathan Willms.) twins are unique to monochorionic twin placentas. However, the number, size, and direction of these seemingly haphazard connections vary markedly (Fig. 45-18). In one analysis of more than 200 monochorionic placentas, the median number of anastomoses was 8, with an interquartile range of 4 to 14 (Zhao, 2013).