the.com/biological clock

means The internal, age-linked sense of time running out — most often used for fertility — that nudges big life decisions along a biological rather than a social timetable.

from The phrase borrows its mechanics from the literal biological clocks scientists identified in the body — circadian rhythms governing sleep and hormones, studied since the mid-20th century. But the popular sense, tied to fertility and the ticking pressure to have children, took off in English usage in the 1970s, as more women entered careers and delayed childbearing, turning a neutral biological concept into a cultural shorthand for a very personal deadline.