Left atrial isomerism (LAI) is a rare syndrome (1 in 10,000-40,000 live births) ⁽¹, that is characterized by the symmetric left-sided aspect of the naturally asymmetric thoracoabdominal organs, as a result of left-right axis disruption. This disorder incorporates a group of cardiac, vascular and visceral anomalies. Multiple combinations are possible and thus prenatal diagnosis can be challenging. The most common findings of this syndrome include several cardiac anomalies (83%) with the reliable finding of two morphological left atrial appendages. Additionally, this syndrome may present congenital heart block (29%), interruption of inferior vena cava with azygos continuation (89%), mispositioned stomach (59%), polysplenia (57%) and midline liver (33%)². Some post-natal diagnostic criteria are not useful in utero since visceral-situs is somewhat inconsistent and difficult to screen, especially the number of spleens or hepatic position. Hence cardiac and vascular anomalies are the most common fetal diagnostic pointers³. In the first trimester, the presence of increased nuchal translucency and bradycardia should raise suspicion of an underlying cardiac anomaly, including those associated with LAI⁴. In one series of 41 cases, the mean gestational age of diagnosis was 18 weeks (range 12-29) ⁽⁵. However, up to the best of our knowledge, no illustrative reports of such an early diagnosis have been published. The prognosis depends mainly, but not exclusively, on cardinal cardiac defects, with the heart block increasing the risk of fetal or perinatal demise³. The global prognosis reported in one large meta-analysis is poor, with 25% of termination of pregnancies, 7% intrauterine and 17% post-natal deaths². The need of surgery was reported in 73% of cases, with a 27% overall death rate during or after the procedure.

Here, we report a case of a thirty-year-old woman with a spontaneous dichorionic/diamniotic pregnancy (3G1P). Uneventful medical or family history, and no consanguinity was reported. This case was sent to our prenatal diagnosis center due to the suspicion of cardiopathy (atrioventricular septal defect and atrioventricular block), associated nuchal translucency above the 99^th centile and jugular lymphatic sacs, in one of the fetuses. At 12^{(3 - 7)} weeks, the ultrasound (Toshiba^® Aplio 400) re-evaluation of fetus A was unremarkable, but fetus B revealed a persistent heart rate below 80 bpm. A thorough evaluation exposed several anomalies, including an atrioventricular septal defect with apparent 2 left atrial appendages, “double vessel” sign, right-sided stomach, discrepancy between the heart rate of the ductus venosus and the aortic arch (Figure 1), and interruption of inferior vena cava with azygos continuation, which was recorded with superb microvascular imaging (SMI) technology (Figure 2). After being elucidated of the meaning of these findings and their prognostic impact, the couple requested selective termination of pregnancy, which was accepted by the institutional board. Concerning the fetus A, the pregnancy occurred unremarkably resulting in a successful full-term birth (3350 g, Apgar score 9/10).

The first trimester fetal bradycardia was crucial to guide the investigation for other LAI-related anomalies. This case highlights the importance of a thorough first trimester morphologic evaluation, especially in twins, where early management of discordant congenital defects benefits perinatal outcomes.