The adage that says mothers tend to baby their youngest children isn’t all fiction. A new study finds that while moms accurately estimate the height of their elder children, they consistently underestimate the height of their youngest. Seventy percent of mothers, researchers found, recall a sudden shift in the size of their youngest child after a new baby was born; the no-longer-youngest child suddenly seemed much bigger. To test whether that was because older children were perceived as larger than reality or younger children were perceived as smaller, and to put numbers to this phenomenon, the scientists asked 77 mothers to mark the estimated height of their 2- to 6-year-old children on a blank wall. While the height estimates of their older children were accurate, mothers guessed that their youngest children were, on average, 7.5 centimeters shorter than their actual height . When a new baby is born, that height guess for the first child jumps and is suddenly more accurate—the “baby illusion” no longer distorts the mother’s estimate. The findings, published online today in Current Biology, could help researchers understand the subconscious factors that influence how parents view their children’s maturity levels or needs.