Three-dimensional sonographic diagnosis of diamniotic conjoined twins

A 26-year-old woman (gravida 2, para 1) was diagnosed as carrying amniotic conjoined fetuses through ultrasound scan at 14 weeks of gestation. She had an unremarkable medical history and no prior history of multiple pregnancy. The tomographic ultrasound imaging (TUI) revealed that the conjoined twins were fused by their bladders, and a patent urachus with allantoic cyst was suspected by sonographic assessment. However, a remarkable dividing membrane was present between the pair (Figure 1A). The umbilical vessels of both twins encircled the cyst and merged into a single umbilical cord (Figure 1B,1C and Video 1), and the umbilical cord was traced until it inserted into the placenta. The twins were fused with omphaloceles (Figure 1D). There were no other abnormalities. One week later, the twins experienced unexplained intrauterine fetal demise. The postnatal appearance was in accord with the prenatal ultrasound findings. The twins were connected only by a short umbilical cord, with no intestinal fusion. The cyst had ruptured during delivery. After delivery, one twin was plethoric and the other was pale (Figure 1E). Chromosomal analysis of the fetuses revealed a normal male karyotype. All procedures performed in this case were in accordance with the ethical standards of the Ethical Review Committee of West China Second University Hospital and with the Declaration of Helsinki and its subsequent amendments. Written informed consent was provided by the patient and her partner for publication of this article and accompanying images. A copy of the written consent is available for review by the editorial office of this journal.

Figure 1 Images in a 26-year-old woman with diamniotic conjoined fetuses. (A) The conjoined twins fused through the BL and a cystic structure, and there was dividing membrane between the 2 fetuses by TUI. The arrows showed the allantoic cysts and arrowheads showed the dividing membranes in different planes. (B,C) The twin’s umbilical vessels surrounded the cyst to assemble one umbilical cord (C: HDlive flow mode). (D) The twins were fused with omphaloceles (HDlive silhouette mode). (E) The conjoined twins were connected at omphalopagus through a short umbilical cord. One was plethoric, and the other was pale. The cyst had ruptured during delivery. BL, bladder; F1, fetal 1; F2, fetal 2; TUI, tomographic ultrasound imaging.

Most authorities have suggested that conjoined twins are monoamniotic with no dividing amniotic membranes between fetuses. However, some monoamniotic conjoined twins with dividing membranes between them have been reported (1,2), representing a unique pattern of conjoined twins. The fetuses described in this case were conjoined with a body stalk anomaly/short umbilical cord, but there were visible dividing membranes. There are several hypotheses about the embryonic pathology of this specific pattern. Kapur et al. proposed the following model that during the folding of the embryonic disk, the blastoderm is wrapped in amniotic cavity, whereas the body stalk, part of the cloacal membrane, allantois, and yolk sac may be extra-amniotic. Therefore, if two embryonic disks are joined in a way that allows the normal development of each amniotic cavity from the dorsal surface of each embryonic disk while being attached to a common yolk sac, but extra-amniotic parts of the disks are fused or become joined during folding, diamniotic conjoined twins will result (3). As the united portion is minimal and there is an obvious dividing amniotic membrane, the conjoined region may be overlooked in early gestation. With the application of new ultrasound technology such as multiplanar image and volume cine technology, we are able to obtain a clear visualization of the parasitic structures from different planes. Due to the HDlive flow mode of three-dimensional (3D) ultrasound, we were able to clearly distinguish that the blood flow was from the allantoic artery of the twins separately, which then wrapped around the cyst, eventually forming only two umbilical arteries. Then, the HDlive silhouette mode showed that the twins were fused with omphaloceles and we could observe the spatial relationship between omphaloceles and the twins. In addition, from the TUI, we could clarify the dividing membrane and the cyst between the twins. As a consequence, 3D sonography was highly useful for the diagnosis of diamniotic conjoined twins and it also facilitated a better understanding both by patients and clinicians. The outcomes of the monochorionic diamniotic cases are often unfavorable due to severe abdominal wall defect or hemodynamic instability resulting from a shared umbilical cord (4). There are several complications in monochorionic twins, including twin anemia polycythemia sequence (TAPS) (5). It is a chronic form of unbalanced fetofetal transfusion through minuscule placental anastomoses in monochorionic twins (5). It leads to anemia in the donor and polycythemia in the recipient (5). During the prenatal ultrasound scans, we only focused on the conjoined twins and did not perform middle cerebral artery (MCA) Doppler. After delivery, TAPS was suspected based on the physical phenotype and their appearances. This may not be a traditional conjoined twin clinical picture but an abnormal connection of umbilical vessels causing transfusion.

Conjoined twins are typically managed via surgical separation following birth (6). Given the poor prognostic outlook for monochorionic conjoined twins (4), many pregnancies end in intrauterine fetal demise. Moving forward, there is a pressing clinical need to explore whether minimally invasive intrauterine modalities such as radiofrequency ablation (RFA) can be safely and effectively translated into fetal therapeutic strategies.

Acknowledgments

The authors thank the engineer, Yanqiong Hou from GE Healthcare for her support with cinematic technology.

Footnote

Funding: This work was supported by the Sichuan Medical Science and Technology Innovation Research Institute Fund (No. 2025ZHCSC008).

Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-2026-0763/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All procedures performed in this case were in accordance with the ethical standards of Ethical Review Committee of West China Second University Hospital and with the Declaration of Helsinki and its subsequent amendments. Written informed consent was obtained from the patient and her partner for publication of this article and accompanying images. A copy of the written consent is available for review by the editorial office of this journal.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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