Hello girls, when is the best time to go for an ultrasound to make sure…

Hello girls, when is the best time to go for an ultrasound to make sure there's an embryo, heartbeat, etc.? I had an ultrasound about a week and a half ago, they put 2-3 and said nothing was visible, only a fetal egg ?‍♀️ Now it will be 6 weeks exactly.