Final answer:
The softening of the uterine isthmus at 6-8 weeks gestation is called Hegar's Sign.
Step-by-step explanation:
During pregnancy, the cervix goes through changes in preparation for childbirth. One of these changes is the softening of the uterine isthmus, which occurs at 6-8 weeks gestation. This softening is known as Hegar's Sign, and it is characterized by the compression and thinning of the lower part of the uterus.