This study included 43 women diagnosed with fetal visceral situs abnormalities by prenatal ultrasound examination between May 2018 and September 2023 at the Prenatal Diagnosis Consultation Center of International Peace Maternity and Child Health Hospital, Shanghai Jiao Tong University School of Medicine. These women were 19–42 (mean, 30.53 $\pm$ 5.56) years old, with gestational ages ranging from 21 to 26 weeks. Follow-up continued until pregnancy was completed, and baseline clinical data and fetal birth information were recorded.

The study was approved by the hospital’s medical ethics committee and all participants provided informed consent.

(1) Situs solitus: normal development of the abdominal and thoracic organ positions.

(2) Situs inversus (formerly complete situs inversus): mirror image of situs solitus.

(3) Heterotaxy/isomerism syndrome (formerly partial situs inversus): left and right isomerism.

(A) Left isomerism (or polysplenia syndrome). In this type, both atria exhibit left atrial morphology (i.e., bilateral left atrial structure), often accompanied by multiple small spleens (polysplenia). The liver and stomach bubble may be in the normal (situs solitus), reversed (situs inversus), or indeterminate position. The inferior vena cava may be interrupted, with an enlarged azygos vein running posterior to the abdominal aorta. This isomerism is often associated with cardiac anomalies, such as atrioventricular septal defects or anomalous pulmonary venous drainage, as well as other organ abnormalities.

(B) Right isomerism (asplenia syndrome). In this type, both atria exhibit right atrial morphology (i.e., bilateral right atrial structure), accompanied by asplenia (absence of the spleen). The liver is typically midline in position, and the stomach bubble may be located on the left, right, or in an indeterminate position. On transverse imaging of the upper abdomen, the abdominal aorta and inferior vena cava are situated on the same side of the spine (ipsilateral arrangement). Right isomerism is often associated with complex cardiac anomalies, such as having a single ventricle or transposition of the great arteries, as well as other organ abnormalities.

Statistical analysis was performed using IBM SPSS Statistics for Windows, version 25.0 (IBM Corp., Armonk, NY, USA). Measurement data were expressed as mean ($\overline{X}$), standard deviation (SD), and percentiles, whereas count data were presented as N (%). Fisher’s exact test was employed to compare the differences in fetal visceral abnormalities accompanied by cardiac malformations. A two-sided p-value 0.050 was considered significant.

The study included 43 cases of fetal visceral situs abnormalities, with an estimated incidence of 0.59‰ (43/72, 499). The pregnancy outcomes included induced labor in 51.16% (22/43), normal delivery in 32.56% (14/43), and lost to follow-up in 16.28% (7/43). Natural conception accounted for 90.70% (39/43), singleton pregnancies for 97.67% (42/43), and indeterminate fetal sex for 65.12% (28/43), which was attributed to being lost to follow-up or the inability to determine the fetal sex after induced labor. Amniocentesis was performed in 48.84% (21/43) of the cases, with chromosomal karyotype and chromosomal microarray analyses yielding negative results. Among these cases, 15 underwent whole-exome sequencing (WES), and the genetic findings were as follows: 1 case carried a suspected pathogenic variant in primary ciliary dyskinesia (PCD)-related gene (maternal autosomal recessive inheritance pattern), which resulted in normal delivery; 4 had variants of uncertain significance (VUS) in PCD-related genes, which included 1 case of PCD type 3 (autosomal recessive inheritance) with pregnancy termination and 3 live births (including 2 maternal VUS and 1 NAH9 gene variant); 1 case of Holt–Oram syndrome (HOS, autosomal dominant inheritance) with maternal VUS (likely benign), resulting in pregnancy termination; and 9 cases not showing clinically significant variants on WES (Table 1).

Our analysis of 43 cases of fetal visceral situs abnormalities revealed distinct patterns of cardiac anomalies between situs inversus and heterotaxy syndrome. The cohort comprised 20 (46.51%) cases of situs inversus, among which only 1 (5.00%) exhibited additional cardiac structural anomalies (pulmonary stenosis, complete transposition of great arteries, or ventricular septal defect). In contrast, all 23 cases of heterotaxy syndrome (53.49%, consistent with cardiospheric syndrome) presented with cardiovascular malformations (100.00%, 23/23), which included left (43.48%, 10/23) and right (56.52%, 13/23) isomerisms. The most prevalent anomalies were single atrium/ventricle (56.52%, 13/23), double-outlet right ventricle (34.78%, 8/23), and pulmonary stenosis (30.43%, 7/23). Statistical analysis showed significant differences in concomitant cardiac anomalies between the groups (p 0.001; Tables 2,3,4).

Complete situs inversus, also referred to as total situs inversus, is a condition characterized by a “mirror-image” configuration of the thoracic and abdominal organs relative to their normal anatomical positions. This condition is also known as mirror-image dextrocardia. If complete situs inversus is not associated with organ or system malformations, most of the fetuses affected have a good prognosis. However, if accompanied by complex cardiac malformations, the prognosis is poor, which is associated with significant surgical challenges and complications.

Following the prenatal ultrasound screening guidelines [7], the multiplanar continuous scanning method is employed during routine prenatal ultrasound examinations. Initially, the fetal orientation is determined, which is key to identifying the location of the fetal viscera. The fetal orientation was determined based on the relationship between the fetal head and spine, with clockwise rotation indicating the left to right side of the fetus in the cephalic presentation and counterclockwise rotation indicating the left to right side in the breech presentation. The standard transverse section of the fetal abdomen was used to observe the positions of the fetal stomach bubble, abdominal aorta, and inferior vena cava.

Fetal orientation determination methods: in the left occiput anterior position, the spine is on the left, and the stomach bubble is in the far field. In the right occiput anterior position, the spine is on the right, and the stomach bubble is in the near field. In the left sacrum anterior position, the spine is on the left, and the stomach bubble is in the near field. In the right sacrum anterior position, the spine is on the right, and the stomach bubble is in the far field. If the fetal position is suboptimal (oblique or transverse), maternal movement is encouraged, and re-examination is performed when the fetal position is suitable.

Between May 2018 and September 2023, our center correctly diagnosed 20 mid-trimester fetuses with situs inversus (no missed or misdiagnosed cases). This achievement can be attributed to several factors. First, 8 of the 20 cases (40.00%) were referred after initial external diagnosis, reflecting our center’s role as a regional referral hub. Second, all patients underwent mandatory screening supervised by 2 senior ultrasonographers, both holding prenatal diagnostic certification and associate/full professor titles. Finally, real-time consensus verification was implemented for every case.

This study included 43 fetuses with visceral situs anomalies, among which 20 (46.51%) were diagnosed with complete situs inversus totalis. Notably, only 1 of these fetuses (5.00%) presented with complex cardiac anomalies, consistent with previous literature reports [8]. When fetal situs inversus is detected during prenatal ultrasound examination, a thorough evaluation, including ventricular looping patterns, atrioventricular connections, ventriculoarterial alignments, and spatial relationships of the great arteries, must be conducted to identify any associated intracardiac malformations. To assess potential extracardiac anomalies, a comprehensive fetal scan should also be performed.

Fetal heterotaxy syndrome, also known as partial situs inversus, may involve only the abdominal or thoracic organs. When the viscera are in the normal position, but the heart is positioned in the right thorax, this condition is referred to as dextroversion. Conversely, if the viscera are arranged in a mirrored configuration, but the heart is located in the left thorax, this condition is known as levoversion. The International Nomenclature Committee for Pediatric and Congenital Heart Disease recommends classifying these conditions as visceral ectopia, which is also known as cardiosplenic syndrome [9, 10].

Cardiosplenic syndrome is mainly divided into 2 types [11]: The first type is polysplenia syndrome, which is characterized by having multiple spleens, bilateral left atrial appendages (left isomerism), bilobed lungs (morphologically left lungs), and interrupted inferior vena cava. It is often associated with atrioventricular septal defects, double-outlet right ventricle, and other complex cardiac malformations, sometimes accompanied by heart block [12]. The other type is asplenia syndrome, which is characterized by the absence of the spleen, bilateral right atrial appendages (right isomerism), trilobed lungs (morphologically right lungs), and associated intracardiac malformations, such as having a single atrium, single ventricle, double-outlet right ventricle, and atrioventricular septal defects. In partial situs inversus, extracardiac malformations often include intestinal malrotation and abnormalities in the position and number of spleens [13, 14]. The prognosis of partial situs inversus depends on the type and severity of the accompanying intracardiac and extracardiac malformations.

This study identified 23 cases of fetal visceral heterotaxy, all presenting with cardiovascular malformations (100.00%), which corresponds with published reports on the co-occurrence rates of these conditions [15]. The most common cardiac anomaly observed was a single atrium/ventricle (56.50%, 13/23), followed by double-outlet right ventricle (34.78%, 8/23) and pulmonary stenosis (30.43%, 7/23). Analysis by isomerism type revealed left isomerism in 10 (43.48%) cases, including 3 with a single atrium/ventricle, whereas right isomerism occurred in 13 (56.52%) cases, including 10 with a single atrium/ventricle. Notably, both groups had the same number of cases with double-outlet right ventricle (n = 4 each). This result is basically consistent with the reported incidence rate of fetal visceral ectopia combined with cardiovascular malformations [16, 17].

Fetal partial visceral ectopia often presents with atrial isomerism, which highlights the importance of determining the left and right atria. The morphology of the atrial appendages is the most reliable basis for distinguishing between the left and right atria. In the four-chamber and short-axis views of the great arteries, the left atrial appendage usually appears finger-like, with a narrow base and slender shape, whereas the right atrial appendage is cone-shaped, with a broad base and shorter, thicker shape. However, given the limitations of ultrasound technology, prenatal diagnosis of some signs of cardiosplenic syndrome remains challenging. Therefore, in addition to mastering the normal fetal anatomy, ultrasound physicians should employ a systematic scanning protocol to not only observe structural abnormalities but also clarify the relationship between fetal presentation and visceral position in order to avoid missing rare cases.

PCD, also referred to as Kartagener syndrome [18, 19], is associated with recurrent respiratory infections after birth and reduced fertility in men. In this study, 15 cases with negative chromosomal karyotype were subjected to WES and microarray analysis. The results revealed 1 case with a suspected pathogenic variant of PCD-related genes (normal delivery) and 4 with variants of uncertain significance in PCD-related genes (1 induced labor and 3 normal deliveries). A report [20] suggests that PCD is a monogenic disorder with a low incidence, but with a genetic predisposition. Therefore, to increase the detection rates of PCD or other genetic abnormalities, WES is recommended for pregnant women with fetal visceral situs and normal chromosomal results, enabling early detection, treatment, and intervention.

HOS, also known as atriodigital dysplasia syndrome or heart–hand syndrome, is a rare autosomal dominant monogenic disorder [21]. Typical clinical manifestations include congenital cardiovascular malformations and varying severity of unilateral or bilateral upper limb dysplasia. Common cardiovascular malformations include atrial and ventricular septal defects, patent ductus arteriosus, and tricuspid atresia. In this study, WES identified HOS in 1 of the 15 cases, which involved right isomerism with a single atrium, common atrioventricular canal, single outflow tract, and anomalous pulmonary venous drainage. No upper limb structural abnormalities were observed, and the pregnancy was terminated.

In this study, fetal visceral position abnormalities accounted for 0.59‰ (43/72,499) of the cases, significantly higher than the previously reported rates [1]. This discrepancy may be attributed to two factors: First, the study period (2018–2023) coincided with the coronavirus disease 2019 (COVID-19) pandemic, during which increased rates of first-trimester severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection were observed. A previous study has suggested the potential contribution of viral infections to abnormal fetal visceral development [22]. Second, as a regional fetal medicine referral center, our hospital’s case selection may be biased toward more complex or abnormal cases.

This study has the following limitations: (1) Challenges in prenatal follow-up management due to the referral-based nature of our center’s cases resulted in substantial lost-to-follow-up instances. (2) The rarity of fetal visceral heterotaxy limits the availability of reference literature for more in-depth comparative analysis. (3) The current evaluation is restricted to prenatal ultrasound diagnostic value without comparative studies with fetal magnetic resonance imaging (MRI) or autopsy findings (the latter remaining the gold standard for diagnosis). Future research should focus on: establishing a multicenter collaborative network to improve follow-up systems; conducting multimodal diagnostic comparisons integrating ultrasound, MRI and autopsy; developing a comprehensive diagnostic framework incorporating genetic evaluation.