Carbonation doesn't change the gravity, but the bubbles can mess up your reading. Just de-gas the sample by pouring it back and forth between two glasses until it stops foaming.
Remember one unassailable statistic, as explained by the late, great George Carlin: "Just think of how stupid the average person is, and then realize half of them are even stupider!"
"I would like to die on Mars, just not on impact." Elon Musk