gestational age accuracy?question

sooo i haven’t had any ultrasounds AT ALL since 20 weeks when i had my anatomy scan. for those of you that have had ultrasounds, is the GA just a guess at this point since they’re bigger? they used the ultrasound machine to check the amniotic fluid and im 36 weeks tomorrow but noticed the GA was 39 weeks and 3 days and i was like WHOOOAA. lol so i’m just wondering for those of you who have had regular ultrasounds since, is this just a guess or am i going to “have this baby in like 4 days?” 😅