Contrast enema can detect small bowel atresia (SBA) in patients with microcolon. In contrast, there is no ultrasonography (US) microcolon criteria for identifying SBA. With this in mind, the purpose of this study was to assess the accuracy of the US in making the diagnosis of SBA, specifically in the colon, relative to a gold standard (surgical or clinical data). About 46 infants less than or equal to 7 days old underwent US. Surgery (n = 15), confirmed the diagnosis of SBA in all 15 infants in the trial group (n = 15). Clinical or surgical follow-up revealed that 15 infants in the control group did not have SBA but other gastrointestinal issues. The control group consisted of 16 healthy newborns who had not yet experienced any GI distress. Colonic gas was looked for, and the colonic diameter was measured. US parameters were compared between the study group and the other 2 groups using statistical analysis. Using receiver operating characteristic analysis, the researchers were able to determine the best colonic diameter cut-off value for identifying SBA cases. The median colonic diameter in the study group was 0.5 centimeters (IQR: 0.5-0.6 centimeters), which was significantly smaller than the median colonic diameter in the groups with and without SBA (0.9 cm; IQR, 0.8–1.2 cm) (P <  0.001) and in the control group (1.2 cm; IQR, 0.8–1.35 cm) (P < 0.001). Optimum cut-off value for diagnosing SBA was 0.65 cm (sensitivity, 90.3%; specificity, 86.7%; accuracy, 89.1%). Accuracy in US-based SBA diagnosis was highest when both microcolon and gas-negativity were included together (91.3%). Microcolon is the correct term for a colon that is less than 0.65 cm in diameter; using ultrasound in conjunction with gas-negativity is the gold standard for diagnosing SBA.