The 580EX II has a tendency to have a problem with the hotshoe contacts. If one of the contacts has a poor connection the flash will revert to TTL instead of ETTL. TTL is only used on film cameras and will cause problems on Digital SLR's.
Make sure the contacts are clean and that the flash is fully seated in the hotshoe. If it is under warranty, Canon will install new "pointy" contacts in your flash that work better than the old "rounded" contacts.
I have no way of knowing how familiar you are with Canon flash.... However, just in case you are relatively new to it and in addition to Mike's excellent reply I'd note several things:
Canon flash in ETTL will act as "fill" whenever the camera is set to any auto exposure mode such as Av, Tv or P. In other words the camera will set the exposure according to the ambient light, and the flash will fire at about 1.5 or 1.7 stop reduced power to just add some fill light and open up shadows. This is aside from or in addition to any Flash Exposure Compensation (FEC) you set using the dial on the back of the flash.
However whenever the camera is set to M or Manual exposure with the flash set to ETTL, the flash will be treated as "full" or the only light source. The camera will ignore ambient lighting conditions and the flash will fire without any reduction in output other than the metering of the camera, unless you dial some in. Depending upon the camera's exposure settings, it is possible to simply be too close to the subject when using the flash this way.
And, if you have pressed the button in the center of the rear dial and turned the dial to show "-1/3" on the LCD while the flash is in ETTL mode, you are not at 1/3 power (which would be a big reduction). This is Flash Exposure Compensation (FEC) and that readout is in thirds of a stop. So instead you have actually only reduced the flash's output down from full power by 1/3 stop (i.e., a very slight reduction... in fact the very least amount you can dial in).
If using the flash this way very close, such as might be the case with a small newborn (depending upon lens focal length), there's a chance of way too powerful a flash. Check the distance scale on the rear of the flash. (Need to know the distance to the subject? Use the distance scale on your lens when it's in focus.... If there is a scale on the lens. Not all have one.)
If the flash's distance scale (across the bottom of the LCD screen in ETTL mode) shows you are too close, that the flash simply can't reduce it's output enough, stop your lens down to allow shooting closer. Or reduce your camera's ISO. Or do some of each.
If you don't want to change those settings, perhaps because you want to use specific ISO and lens aperture, another trick is to set the flash's zoom manually to the widest possible setting and perhaps even pull out and use the wide angle diffuser panel too. This will significantly reduce the flash's reach. Recheck the distance scale shown on the flash's LCD screen, to be sure it's reduced enough.
Yet another thing that reduces the flash's reach significantly and can be helpful for close up shooting is High Speed Sync. Set the flash to HSS and then use a faster shutter speed, above the camera's usual flash sync speed. This can significantly reduce the flash's reach.
All these can be done while still using ETTL and the camera in M exposure mode. You also can still make some fine adjustments of the flash's effect with Flash Exposure Compensation (FEC) if you wish, via the dial on the back of the flash.