The short story is that all pregnancy tests are checking for a pregnancy hormone called Human chorionic gonadotropin (HCG).

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It follows the lives of individuals at the key stages in the human life cycle: birth, childhood, adolescence, pregnancy, adulthood, old age and death.

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HCG, human chorionic gonadotropin, is a hormone produced by the placenta during pregnancy.

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For much of human history, a woman's reproductive life was dominated by pregnancy and breast-feeding, both of which block menstruation.

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