1. Department of Ultrasound Diagnosis Gansu Provincial Maternity and Child‐care Hospital Lanzhou, Gansu Province P. R. China
2. Department of Ultrasound Gansu Provincial Ultrasound Imaging Clinical Medicine Research Center Lanzhou, Gansu Province P. R. China
3. Department of Ultrasound Diagnosis Anding District Maternal and Child Health Hospital Dingxi, Gansu Province P. R. China