If your MacBook Pro has an internal battery - seek service as they are supposed to self-calibrate and not need any sort of maintenance.
For reference: Apple Portables: Calibrating your computer's battery for best performance
If your model has a removable battery, those are older and the Mac needs you to periodically run a calibration where you charge it to full and leave it connected to power for two or so hours. Then disconnect power and run it down over a day or two - never connecting to power. Eventually, it will go to sleep and then you leave it for 4-5 hours in sleep to finally reach the low voltage shut off.
This will recalibrate the sensor for full and empty and perhaps fix the OS reporting a different charge level than reality. As the battery ages, the "guess" to how much charge capacity is left at a certain voltage level changes. This calibration run should be done every 6 to 10 months if you care to have a more accurate OS battery estimate. Of course, if there is a failure in the battery or the hardware - that will prevent this from working and you will again be seeking service for a repair.
If you are not near a support center - you could try the discharge run on a newer Mac with the newer battery.