My wife is 12 weeks 2 day pregnant. her gyn. did vaginal and normal ultrasound. he said he thinks the baby is a boy. how accurate the result?

Pretty good, but…: I would not decorate the nursery just yet. If well visualized, the gender of the baby can be accurately determined. However, 12 weeks is early. The baby is quite small. Reevaluation at the fetal anatomy screening between 18 and 21 weeks would be helpful. Even then, sometimes it's hard to tell, and people have been surprised at delivery.