Final answer:
The detection of human chorionic gonadotropin (hCG) in blood or urine is a probable indication of pregnancy, as it is critical for maintaining elevated levels of progesterone for embryo implantation and growth.
Step-by-step explanation:
The presence of human chorionic gonadotropin (hCG) in the blood or urine is a probable sign of pregnancy. This hormone is produced by the chorion of the zygote, and one of its essential roles is to preserve the corpus luteum, allowing it to continue secreting progesterone. The maintenance of elevated levels of progesterone is crucial for the successful implantation and growth of the embryo. In a non-pregnant woman, the absence of fertilization leads to the degeneration of the corpus luteum, causing progesterone levels to decrease and the endometrium to shed, which initiates the menstrual cycle.