The narrowest part of the uterus is the isthmus of the uterus. The isthmus of the uterus is anatomically called the anatomical ectocervix, which is clinically the narrowest part of the uterus, and its lower end is at the endometrium, which is converted to the endocervical layer, so it is called the histological endocervix. The isthmus is the junction between the endometrial layer and the endocervical layer, and is clinically divided into the endocervical isthmus, and the anatomical and histological endografts. The narrowest part between the body of the uterus and the cervix is 1 cm in non-pregnancy. the isthmus is not a point, but a surface, with an area of about 1 cm^2. With pregnancy and delivery, the isthmus of the uterus expands, and after pregnancy it becomes the lower segment of the uterus.