The shape and size of the uterus

The normal uterus is shaped like an inverted pear, flat in front and slightly convex in the back. It is wide at the top and narrow at the bottom, and can be divided into two parts of different sizes. The upper part of the uterus is wider and triangular, called the uterine body, and its upper end is arched and called the uterine fundus. The two sides of the uterine fundus are the uterine horns, which are connected to the fallopian tubes.

The uterine horns that have not yet fully expanded are darker funnel-shaped. When fully expanded, the openings of the fallopian tubes can be seen on both sides of the top, which are mostly round, but also star-shaped or oval. The lower part of the uterus is cylindrical and is called the cervix. The size of the uterine body and the ratio of the uterine body to the cervix vary with development and age. The ratio of the uterine body to the cervix is ​​1:2 in infants and 2:1 in adults. The adult uterus is a hollow organ with a uterine cavity capacity of about 5 ml. The uterus of a normal adult woman weighs about 50 g, is about 7 to 8 cm long, about 4 to 5 cm wide, and about 2 to 3 cm thick.

The narrowest part between the uterine body and the cervix is ​​called the isthmus of the uterus. It is about 1 cm long during the non-pregnant period, and its lower end is connected to the cervical cavity. The upper end of the isthmus of the uterus is also called the anatomical internal os because it is anatomically very narrow; the lower end of the isthmus is also called the histological internal os because the mucosal tissue here changes from the endometrium of the uterine cavity to the endometrium of the cervix. The isthmus gradually expands during pregnancy, especially when labor is approaching, it expands to 10~12 cm in length, which can accommodate the descent of the fetus and form the lower uterine segment.