Definition - What does Uterine Isthmus mean?
The uterine isthmus is a narrow part of the uterus that connects the body of the uterus to the cervix—the lowest part of the uterus. It is about 7mm in size and increases to about 25 mm during the early weeks of pregnancy. It makes the thinnest portion of the uterus during pregnancy and forms the lower uterine segment. Being thin, it is also more likely to rupture during labor.
FertilitySmarts explains Uterine Isthmus
The uterine isthmus also provides a non-sensitive prediction of pregnancy; the isthmus becomes soft, thin, and flexible such that uterus and cervix seem to be two separate regions when compressed between fingers of two hand—two fingers placed inside the vagina and the fingers of the other hand placed over the pelvic area. This is due to increased vascularity and because the fetus does not yet fill the uterine cavity.
This finding is known as Hegar’s sign. It is present between 4th and 12th week of pregnancy. However, the absence of Hegar’s sign does not exclude pregnancy.