Current guidelines recommend an F-echo for mothers with CHD. The guidelines do not take into account if a LII-US was felt to be normal [8, 9]. There is increasing data suggesting that a normal second-trimester screening ultrasound is unlikely to miss critical CHD [13,14,15,16,17,18,19,20,21]. There are minimal data evaluating the utility of F-echo in mothers with CHD when a LII-US read was normal. In this study, a normal LII-US did not miss any critical CHD. LII-US did miss mainly septal defects and minor valve issues, but this was also true of the F-echo.

These findings are consistent with multiple recent studies documenting the accuracy of second-trimester screening ultrasounds when compared to F-echo in diagnosing CHD and most importantly critical CHD [13,14,15,16,17,18,19,20,21]. These previous studies have noted an approximately 2–3% missed CHD diagnosis rate on the second-trimester screening ultrasound when compared to an F-echo in the current era [13, 18]. The possibility of a missed critical CHD when there was a prior normal second-trimester screening ultrasound was < 0.5% in large population studies [13, 19, 20]. The estimated number of F-echo needed to be performed to detect a missed critical CHD ranged from 249 to 750 [13, 20].

Despite the fact that no critical CHD was missed on LII-US, there still remained patients that were diagnosed with CHD in the postnatal period. The sensitivity for F-echo may range as low as 20% to as high as 80% for revealing minor CHD [23,24,25]. Most of the missed CHD in this study dealt with septal defects or minor valvular abnormalities, which is consistent with previous F-echo studies [23,24,25,26]. There are also certain types of CHD that are difficult or impossible to diagnose on F-echo such as a patent ductus arteriosus, coarctation of the aorta, and septal defects. These are known limitations to F-echo.

We acknowledge that the purpose of F-echo is not only for diagnosis, but also for subsequent counseling of families when a CHD is diagnosed. In this study, two CHD diagnoses were correctly made that the LII-US missed, a vascular ring and mild aortic root dilation. However, these prenatal diagnoses did not alter the overall postnatal cardiac care. Both patients are asymptomatic and are being followed conservatively. Conversely, this must be weighed against the reassurance of a false-negative F-echo or the anxiety that may be induced for a false-positive F-echo report. We concede that the vascular ring may never have been diagnosed postnatally, unless the patient developed symptoms. Treatment options are variable for asymptomatic vascular rings with nonintervention being one possibility, as is the case for this patient [27,28,29,30]. In addition, the patient with aortic root dilation would have had a pTTE regardless of the F-echo because of the family history per guidelines, so this did not affect overall clinical management [31].

Six patients had false-positive results on the F-echo, five patients in this study had normal F-echo, but were diagnosed with CHD in the postnatal period, and three patients had CHD concerns on the F-echo, but the pTTE diagnosed a different CHD. Of note, the referral indication was a murmur for all five of the patients with a normal F-echo. False-positive results are a known limitation of F-echo [16]. Increased maternal anxiety due to a fetal diagnosis of CHD is well reported and thus a false-positive F-echo would be a negative experience for the family [32,33,34,35].

Another factor to consider whenever ordering additional ultrasound testing is bioeffects and safety. Even though ultrasound emits minimal bioeffects, all ultrasound studies should follow the “as low as reasonably achievable (ALARA)” principle [10]. This principle affects both evaluating imaging modality and what exams are absolutely necessary but also length of scans. If CHD can be adequately ruled out with a LII-US, then by following ALARA, an F-echo might not be deemed necessary.

Due to these issues, an alternative strategy may be to reassure the family that it is extremely unlikely that a critical CHD is present if the LII-US was normal. There is no question that if there are cardiac concerns or even uncertainty on the LII-US, an F-echo should be performed [8, 9]. If there are cardiac concerns after the fetus is born, then a pTTE may be performed to reassess the situation [36]. This occurred in the five patients that had a normal F-echo, but subsequently had a CHD diagnosis postnatally. This possible change in strategy is also reflected in the slight changes present from the initial F-echo guidelines published in 2014 versus the updated guidelines published in 2023 [8, 9]. Some previous indications that were listed as may be indicated or probably indicated have been changed to not indicated or may be indicated, respectively. As stated by the most recent guidelines, “Some conclude that a well-executed obstetric scan is cost-effective compared with referring these higher risk pregnant patients directly for fetal echocardiography. However, this conclusion depends heavily on the quality of the screening ultrasound and its interpretation, and the cost–benefit ratio of screening is related to the sensitivity, specificity, and posttest probability as described earlier.” [9]. As technology and comfort level with evaluating cardiac issues improves over time in the obstetric and maternal–fetal field, one would assume the LII-US and possibly the regular second-trimester screening ultrasound will continue to improve in detecting CHD [18].

There are multiple limitations of this study. This was a retrospective study with all the inherent shortcomings of such a design. The study assessed a single center, and the overall numbers were thus small, despite evaluating an 8-year time period. Maternal CHD diagnosis was heterogeneous. Twenty-three patients were unaccounted for postnatally. Of those patients, 22 had normal F-echo and one had concerns for a small muscular ventricular septal defect, so it is extremely unlikely critical CHD was missed, though it is possible that other minor CHD diagnoses were overlooked. LII-US were from a variety of clinics, but all located within a major metropolitan area and thus the generalizability of these results to other settings needs to be evaluated. The findings are all based on LII-US reports since access to the images were unable to be obtained, hence this study could not delineate if LII-US accuracy was due to image quality or interpretation. Similarly, body mass indexes were not available for the mothers, so no comment can be made if diagnoses were possibly missed due to difficult images due to maternal body habitus. The educational degree and experience level of the sonographer or physician performing the LII-US is unknown, thus no comment can be made if better or worse detection of CHD would have occurred based on these factors. This study did not evaluate the accuracy of an LII-US if a cardiac concern was present since this was not the purpose of this study. This study did not evaluate the accuracy of a standard second-trimester obstetric ultrasound that is performed for non-high-risk pregnancies so no comment can be made in that regard.