TTN is a common cause of respiratory distress in newborns caused by retained fetal lung fluid and consists of a period of rapid breathing that usually resolves within 24–72 h [25]. LUS is a non-invasive tool that is increasingly used in the diagnosis of TTN. It allows visualization of the neonate’s lungs and real-time assessment of conditions like TTN. It also helps exclude other lung diseases and guides further management [26].

Clinical evaluation of cardiac status is important to assess sequelae. EC can be used to noninvasively measure extravascular lung water index in TTN. By tracking changes in thoracic electrical bioimpedance during the cardiac cycle, lung edema and decreased lung water content over time as the condition resolves can be continuously monitored. This technique may allow quantification of disease severity and treatment response without radiation exposure [22].

In the current study, there was no significant difference between both groups as regards gestational age, gender, Apgar 1 and 5 min, anthropometric measurements, or antenatal risk factors, while, as regards mode of delivery, there was a statistically significant increase in CS deliveries in TTN group. This came in agreement with Derbent et al. [27], who showed that the proportion of CS in the TTN group was significantly higher. Another retrospective study by Kasap et al. [28] carried out on 95 newborns with TTN and showed that 79% of the patients were delivered by CS.

TFC was significantly higher in TTN group on D1 compared to NLD group. Furthermore, there was statistically significant decrease in TFC on D3 compared to D1 in TTN group. But there was no statistically significant difference between TTN group on D3 and NLD group. This could be explained by the resorption of lung fluids over time in TTN patients.

In agreement with our findings, Bassiouny et al. [29] revealed that TFC within the first 6 h was high. However, TFC at 24 h of ≤ 24 mL/kg and TFC drop rate at 24 h of > 12% are statistically significant discriminators of TTN from non-TTN.

TFC also was positively correlated with Downes’ and TTN scores in the TTN group. These results agree with Paviotti et al. [12], who found that TFC independently correlates with the presence of respiratory distress at birth and at 24 h of life in late preterm and term newborns.

LUS findings demonstrated DLP only in 31 cases, pleural line abnormalities were found in all the cases, while white lung was present in 6 in newborns with TTN. These results agree with Raimondi et al. [24], who concluded that pleural line with no consolidation is a consistent finding in TTN and the presence of a DLP is not essential for the LUS diagnosis of TTN.

As regards LUS, in our present study, there was significant increase in LUS score in TTN group on D1 compared to NLD group. Also, there was significant decrease in LUS score at D3 compared to D1 in TTN group. The difference between LUS score on D1 and D3 in TTN group could be explained that LUS score decreased progressively over time with resolution of TTN. Our results agreed with Pezza et al. [30], who showed that lung aeration score was evaluated and improved over time in TTN patients.

This also agreed with Li et al. [31], who found that LUS scores decreased significantly from day 1 to day 2. They also found that TTN group exhibited significantly higher LUS scores than did the control group. Also, Yoon et al. [22] demonstrated that LUS score for the prediction of TTN had a sensitivity of 67% and specificity of 97%. This also came in agreement with He et al. [32] meta-analysis which evaluated the diagnostic value of LUS for detecting TTN.

Our data showed a significant positive correlation between LUS and both Downes’ and TTN scores in the studied cases on D1 and D3. This can be explained because LUS score correlated with the severity of respiratory distress which was clinically assessed with Downes’ and clinical TTN scores during the TTN course. This observation came in agreement with the study by Raimondi et al. [24], who found a significant correlation between LUS and Silverman score. Both scores decreased progressively over time.

In our present study, there was a significant positive correlation between LUS score and duration of oxygen therapy. This agreed with Gunes et al. [33], who showed positive correlation between LUS and oxygen exposure. Also, Li et al. [31] observed a moderate correlation between the LUS score and respiratory severity score (RSS), which indicates that the LUS score reflects the clinical respiratory severity of neonates diagnosed with TTN.

We also observed a significant positive correlation between LUS and TFC on the first and third days in TTN cases. This came in agreement with Yoon et al. [22], who found that TFC correlated well with ultrasound in the estimation of extravascular lung fluid. As regards FTC and SVV, there was no statistically significant difference between TTN group and NLD group.

To the best of our knowledge, there are few studies in the current literature evaluating the EC in the diagnosis of TTN. Meanwhile, this is the first study to compare EC versus LUS to diagnose and follow up newborns with TTN.

The current study has some limitations; it was a single-center study, sample size was relatively small, the lack of studies about the reproducibility of the EC in neonates, the time points of evaluation can be variable within first 24 h of age and on the 3rd day, and neonates with different respiratory disorders like RDS were not included.