biological clock

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An internal mechanism in organisms that controls the periodicity of various functions or activities, such as metabolic changes, sleep cycles, or photosynthesis.
• noun The time period during which a person who desires to have a biological child, especially a woman approaching the decline of reproductive capability, is still able to do so.
• noun The capacity to be fertile, especially in such a woman.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun biology Any of several mechanisms that regulate the periodicity of biological functions
• noun The progression from puberty to menopause during which a woman can bear children

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an innate mechanism in living organisms that controls the periodicity of many physiological functions