子宫峡部是子宫下后方，靠近子宫颈的部份，此处子宫的肌肉（子宫肌层）较细较窄，子宫峡部的上方和前方是子宫体及子宫底。

子宫峡部在怀孕时会变的很柔软，这称为海格氏征象。

The uterine isthmus is the inferior-posterior part of uterus, on its cervical end — here the uterine muscle (myometrium) is narrower and thinner. It connects superiorly-anteriorly to the complementary parts of the uterus: the body and the fundus.

The uterine isthmus can become more compressibile in pregnancy, which is a finding known as Hegar's sign.