biological clock

Từ điển Việt Anh Việt 4in1 - English Vietnamese 4 in 1 Dictionary

ˌbiological ˈclock BrE AmE noun [singular]
1. technical the system in plants and animals that controls when they sleep, eat, produce babies etc SYN body clock
2. the idea that when a woman reaches a certain age, she will soon be too old to have a baby:
career women who hear the biological clock ticking