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Infant chest compressions should press about one-third of the chest’s front-to-back (AP) depth. In practical terms, that’s roughly 4 cm (about 1.5 inches). This depth is chosen because it provides enough pressure to circulate blood without risking injury to the infant’s soft, flexible chest.

The one-third guideline adapts to the infant’s size, which is why it’s expressed as a fraction of chest depth rather than a fixed number. If you compress much deeper than about 4 cm, the risk of rib or organ injury increases; if you don’t compress enough, you may not achieve effective blood flow.

So the best answer matches the guideline: about 1.5 inches (4 cm), which is at least one-third of the chest depth.