Which sign indicates pregnancy as early as 12 weeks gestation?

Chadwick's sign is characterized by a bluish discoloration of the cervix, vagina, and labia resulting from increased blood flow and vascular congestion in pelvic tissues. This sign can be observed as early as 6-8 weeks of gestation but is often noted by 12 weeks. Its presence is a reliable indicator of pregnancy as the body undergoes significant hormonal changes, promoting increased blood flow to the reproductive organs.

The other options, while they may relate to pregnancy or change with gestational progress, are less specific to the early stages of pregnancy. The uterus becomes palpable around the level of the umbilicus at approximately 20 weeks gestation, and blood pressure typically does not lower until later in pregnancy due to hemodynamic changes. Hyperreflexia is not a standard sign of pregnancy and does not indicate gestational status. Thus, Chadwick's sign is the most pertinent indicator in the context specified.