Which sign refers to the softening and compressibility of the lower uterus during pregnancy?

Hegar's sign is indeed the correct choice as it specifically refers to the softening and compressibility of the lower segment of the uterus during pregnancy. This physiological change occurs as the body prepares for the accommodation of the growing fetus. During a pelvic examination, healthcare providers can assess this sign, which typically becomes noticeable around the sixth to eighth week of gestation.

In contrast, Piskacek's sign refers to a localized asymmetric enlargement of the uterus due to the presence of an implanting embryo, Goodell's sign indicates the softening of the cervix, and Chadwick's sign involves the bluish discoloration of the cervix and vagina due to increased blood flow, which can be observed early in pregnancy. Each of these signs has its own unique characteristics and relevance in the context of pregnancy, but Hegar's sign is the one that specifically highlights the lower uterine segment's changes.