As a teenager, I taught myself to write with my non-dominant left hand. My penmanship became legible if not elegant. I liked the different poems that emerged. Nevertheless, once my identity calcified post-pubescence, I gave up the practice for 17 years—until I started writing with my left hand while breastfeeding my first child.  Once again, I can feel hesitant new neural pathways forming. Once again, I am surprised by what I write, and by what it says about who I am becoming. A few weeks after my son was born, I read Lucy Jones's book Matrescence, in which she cites a recent study showing that the brain changes undergone by biological mothers are comparable in scope and consequence to the brain changes commonly associated with puberty:  “The group compared the brains of twenty-five first-time mothers with twenty-five female adolescents; the brain changes were extraordinarily similar. Each group had reductions in cerebral grey matter volume at the same monthly rate. Both showed changes in cortical thickness and surface area, and depth, length and width of sulcal grooves (the lower part of the Viennetta-like ripples of the brain).” Though this study was small, it confirms Jones’s—and my—subjective experience: Like teenagers, new mothers are engaged in a dramatic, intensive identity-forming process.