Biological clock, is it a specific length of time?

Is there a specific length of time that a woman's body tells her to have babies? Is it the same of most women? does this have to do with hormones, society, or both?

women reach their peak in eairly 20s and it ends in mid 30s...this is the time in which your body is strongest and most prepaired for conception and delivery

It has to do with age. As a woman gets older, there is more chance that her baby will have birth defects. Around the age of 40, these risks increase dramatically, although more and more women, today, are having children later because more can be done to lower the risks.

The risks are slightly higher for a woman who is having her first child over 40.