My wife is 21 weeks pregnant and had a 4D ultrasound and the doctor said the length of the nasal bone could not be observed. Doctor, could you tell me why the ultrasound of a 21-week-old fetus could not show the length of the nasal bone? Is it possible that the baby is not in the right position so the length of the nasal bone cannot be observed, doctor? I look forward to your advice, thank you.