Because of epigenetics. Genes are little instruction manuals for making proteins, but there is a whole second part of genetics related to gene expression. Basically your body uses signals to turn on, turn off, or regulate where or how much of a protein to make. This creates an amazingly complex choreography of gene expression that builds your body the way it is. Many of these signals are coming from other genes, it’s genes regulating genes. But some of them come from the environment.
There is a whole bunch of development going on during pregnancy, and your environment in that case is your mothers body. So as women age the epigenetic signals they side to a baby in the womb are different, and this leads to different types of development.
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