Adaptive acoustic output selection in ultrasound imaging promises to improve adherence to the ALARA (as low as reasonably achievable) principle. Demonstrated imaging sequences have balanced image quality and safety considerations based on transducer channel lag-one coherence to select Mechanical Index (MI), a measure of acoustic output. In this study, data were collected with an adaptive output selection tool on 9 pregnant volunteers to explore factors affecting the recommended output. The placenta, fetal abdomen, and fetal heart in each volunteer were repeatedly scanned. A statistically significant difference in recommended ALARA MI was observed between target structures (p < 0.001). Each target's underlying channel data signal magnitude correlated with the recommended ALARA MI used for scanning, suggesting the brightness of a target affects the selected ALARA MI. These findings imply that continued observance of the ALARA principle during clinical examinations requires adaptive output selection be performed with each new target structure, or each time a transducer or acoustic window shift causes a change in the target brightness,