Discussion
The correct answer is conjoined twins of the monochorionic monoamniotic, thoraco-omphalopagus type. Several sonographic features distinguish this diagnosis from the alternatives listed.
First, the two fetuses maintained a fixed, constant relative position throughout the real-time examination, facing one another, with only independent rotational movement about that fixed point — unlike separate monochorionic diamniotic (MCDA) twins, which move independently of one another and are not bound in a persistent orientation. Discordant growth in MCDA twins also does not produce an inseparable, fixed relationship between fetal poles.
Second, an uninterrupted skin line was seen across the midline of the chest and upper abdomen, with no dividing amniotic septum identified — the combination of monoamniocity and continuity of the body contour between the two fetuses is a key diagnostic criterion for conjoined twins, rather than an artefact of close proximity in a normal monoamniotic pregnancy.
Third, both fetal hearts showed normal, concordant cardiac activity (162 bpm). This argues against TRAP sequence, in which the "acardiac" twin characteristically shows absent or grossly abnormal cardiac activity and structure, with perfusion instead depending entirely on abnormal placental vascular anastomoses from the pump twin.
Finally, a membrane-covered sac was seen herniating through a defect in the anterior abdominal wall of fetus two (measuring 0.95 cm), consistent with an omphalocele, while the abdominal wall of fetus one remained intact. This is distinct from a single fetus with an isolated omphalocele, in which only one fetal pole, one heart, and one set of biometry would be expected — here, two separate fetal poles and two hearts were clearly seen. Omphalocele and other structural anomalies are well recognised associations of thoraco-omphalopagus conjoined twins.
Taken together, the fixed inseparable position, shared skin contour, absent dividing membrane, and site of fusion at the thorax/upper abdomen (with an associated omphalocele in one twin) support first-trimester diagnosis of thoraco-omphalopagus conjoined twins over the other options.
Additionally, the nuchal translucency of both fetuses were taken and they were 2.8 mm (normal limit is 2.5 m) for fetus 2 and 1.5 mm for fetus 1. The points to increased risk of chromosomal anomalies.
Additional images / videos
Two accompanying real-time video clips were also obtained.
Video 1: Real-time transvaginal sweep demonstrating the two fetuses maintaining a fixed relative position and orientation despite probe and maternal movement.
VIDEO
Video 2: Real-time clip demonstrating independent rotational movement of the twin unit as a whole, with concordant cardiac activity in both fetuses.
VIDEO
Given the first-trimester diagnosis above, what is the most appropriate next step in management of this pregnancy?
Additional discussion
Conjoined twins are a rare form of monochorionic monoamniotic twinning, and thoraco-omphalopagus fusion is one of the more common subtypes. Once suspected, management should involve a detailed follow-up scan in the early second trimester (around 14–16 weeks) to define the extent of fusion, assess for shared organs, and perform a dedicated fetal echocardiogram, since congenital heart disease and other structural anomalies are frequently associated. Multidisciplinary counselling — involving maternal-fetal medicine, pediatric surgery, and, where relevant, cardiology — is essential to discuss prognosis and the options available to the parents, given the generally guarded outcome associated with conjoined twinning.
Conclusion
First-trimester transvaginal ultrasound can reliably identify conjoined twins using a small set of reproducible signs: a persistent, fixed relative position of the fetal poles that does not change with movement, an uninterrupted skin contour across the presumed line of fusion, and absence of a dividing membrane in a monoamniotic sac. Recognising these features early — as in this case, at 11 weeks — allows timely referral for detailed anatomical and cardiac assessment and gives parents the opportunity for informed counselling and decision-making earlier in pregnancy.
Conflicts of Interest:
The authors declare no conflict of interest.
References
Mathew, R.P.; Francis, S.; Basti, R.S.; Suresh, H.B.; Rajarathnam, A.; Cunha, P.D.; Rao, S.V. Conjoined twins – role of imaging and recent advances. J. Ultrason. 2017, 17(71), 259–266.
Bhatia, H.; Aggarwal, P.; Devkota, S.; Lamichhane, S. Thoraco-omphalopagus conjoined twins: comprehensive evaluation with sonography and MRI in first trimester—a rare imaging diagnosis. BJR Case Rep. 2025, 11(1), uaae045.
Additional discussion
Conjoined twins are a rare form of monochorionic monoamniotic twinning, and thoraco-omphalopagus fusion is one of the more common subtypes. Once suspected, management should involve a detailed follow-up scan in the early second trimester (around 14–16 weeks) to define the extent of fusion, assess for shared organs, and perform a dedicated fetal echocardiogram, since congenital heart disease and other structural anomalies are frequently associated. Multidisciplinary counselling — involving maternal-fetal medicine, pediatric surgery, and, where relevant, cardiology — is essential to discuss prognosis and the options available to the parents, given the generally guarded outcome associated with conjoined twinning.
Conclusion
First-trimester transvaginal ultrasound can reliably identify conjoined twins using a small set of reproducible signs: a persistent, fixed relative position of the fetal poles that does not change with movement, an uninterrupted skin contour across the presumed line of fusion, and absence of a dividing membrane in a monoamniotic sac. Recognising these features early — as in this case, at 11 weeks — allows timely referral for detailed anatomical and cardiac assessment and gives parents the opportunity for informed counselling and decision-making earlier in pregnancy.
Conflicts of Interest:
The authors declare no conflict of interest.
References
Mathew, R.P.; Francis, S.; Basti, R.S.; Suresh, H.B.; Rajarathnam, A.; Cunha, P.D.; Rao, S.V. Conjoined twins – role of imaging and recent advances. J. Ultrason. 2017, 17(71), 259–266.
Bhatia, H.; Aggarwal, P.; Devkota, S.; Lamichhane, S. Thoraco-omphalopagus conjoined twins: comprehensive evaluation with sonography and MRI in first trimester—a rare imaging diagnosis. BJR Case Rep. 2025, 11(1), uaae045.
