Uterine isthmus

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.