You have to degas it by pouring it between some glasses a couple of times, or just leave it out. I think that the carbonation would suspend the hydrometer and affect the reading.
Even beer poured right out of the fermenter will have a small amount of Residual Co2 present in the beer. The temperature in which you take your readings is important since colder beer will retain more Co2 than warmer beer.
'Give a man a beer and he'll waste an hour, teach him to brew beer and he'll waste a lifetime'.