The earliest reliable sign of pregnancy is a positive hCG test because human chorionic gonadotropin (hCG) is produced by the developing placenta shortly after implantation, typically detectable in blood or urine within 7-10 days post-conception. Clinical symptoms like breast tenderness, missed period, and nausea are subjective and appear later, whereas hCG detection provides an objective biochemical confirmation of pregnancy. Therefore, the positive hCG test precedes physical and symptomatic signs, making it the most dependable early indicator.

Reference: Williams Obstetrics, Cunningham, 25th Edition.