around 6 weeks through 陰道 ultrasound
around 8 weeks through 肚皮 ultrasound
But not every doctor will check your baby by 陰道 ultrasound, so you can't see the bb through 肚皮 ultrasound before 8 weeks is normal. So it is better to have check up around or after 8 weeks, otherwise you may 自己嚇自己.