Pregnancy tests detect human chorionic gonadotropin (hCG), a hormone produced by the placenta shortly after the embryo attaches to the uterine lining. The presence of hCG in urine or blood confirms pregnancy because it is not normally found in significant amounts in non-pregnant individuals. Unlike progesterone, estrogen, or luteinizing hormone, hCG levels rise rapidly in early pregnancy**, making it a reliable marker for pregnancy detection. This hormone supports the corpus luteum and maintains progesterone production during the first trimester.

Reference: Williams Obstetrics, Cunningham FG, 25th Edition.