What sign indicates softening of the cervical isthmus during pregnancy?

Hegar's Sign is the correct indication of softening of the cervical isthmus during pregnancy. This sign is assessed during a pelvic examination and typically occurs around the 6th to 8th week of gestation. The softening is an important physiological change that helps accommodate the growing uterus and reflects hormonal changes taking place in the body.

Chadwick's Sign refers to the bluish discoloration of the cervix and vagina that occurs due to increased blood flow, typically noticeable around the 6th week of pregnancy. Goodell's Sign reflects the softening of the cervix itself, which occurs due to increased vascularization and hormonal influences but does not specifically address the isthmus. McDonald's Sign is related to the flexibility of the uterus, specifically the ease of manipulation of the uterus through the cervix, usually assessed later in pregnancy. Each of these signs provides valuable information about the changes happening in the body during pregnancy, but Hegar's Sign specifically identifies the softening of the cervical isthmus, making it the most relevant choice for this question.