Ms. Conaboy is a journalist specializing in health and the author of the forthcoming book “Mother Brain: How Neuroscience Is Rewriting the Story of Parenthood,” from which this essay has been adapted.Around the time that Mimi Niles became a mother, an upstairs neighbor in her New York City apartment building had twins. When the two women ran into each other in the hallway or on the sidewalk, Ms. Niles would ask the neighbor how she was faring. “Fabulous,” Ms. Niles remembered her saying. “I’m so happy.” Ms. Niles was dumbfounded. She was not feeling fabulous in new motherhood. She was...