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Humans, like virtually all other living organisms, are hardwired with a biological clock of sorts. Thanks to the hypothalamus inside the brain, our bodies are cued to our wake/sleep cycle and other regulatory mechanisms. This is something we share with trees, bears, funghi, and just about every living thing you can name. A flower is triggered to release more pollen at a certain time. Deciduous forests are primed to shed their leaves as part of the yearly cycle of the seasons. As Trace explains here, it's an example of convergent evolution-an adaptation shared by myriad forms of life.
In humans, scientists are always learning more and more about men and women's biological clocks. For instance, a study published in Human Reproduction concluded that women are most fertile in their mid 20s. After age 27, the chances of becoming pregnant gradually start to decline. It's quite common for women, during this time, to feel a sense of urgency to get pregnant. The body has a way (release of hormones helps) of letting us know that this is happening.
This natural process is not exclusive to women by any means. Men also experience decreasing levels of fertility. With age, sperm volume and quality decrease, especially after age 40. A 2008 study in Nature found that children of male parents older than 40 were far more likely to be diagnosed on the autism spectrum disorder.
Interestingly, a 2011 study published in Emotion found that women reported fewer "baby crazy" feelings than men after they had the first child. Perhaps women feel a completed sense of satisfaction with one offspring, whereas men want to spread their seed more? More research needed here.