White spot on babies brain in sonogram?

i went today for a sonogram (20 weeks) they didn't tell me anything just the , tho i did notice them taking a LOT of pictures of the brain but thought that was normal... well i happened to have a doctors appointment today and she said they found a white spot on the babies brain... my doctor is now having me go somewhere else in a few weeks because she said that the white spots usually dissolve and its nothing to worry about... well what does the white spot mean? and what if it doesn't dissolve... what happens? does anyone know because my doc. wouldn't give me any straight answers (think she just doesn't want me to worry)