The cervix is divided into anatomy, internal mouth and histological inner mouth. There is a hysterectomy between them. The normal length of the cervix is 2.5-3cm. When used as a gynecological examination, the cervix can successfully pass through the 7th expansion stick. Or there is no obvious abdominal pain during pregnancy and B ultrasound internal width & GT; 0.5 cm, cervical tube length & lt; 2.5 cm, which is helpful for diagnosis. From the 4th to 5th month of pregnancy.