Humans have the largest newborn to adult size ratio of all animals. This is likely due to our uniquely aggressive trophoblast invasion of the maternal uterine spiral arterioles that supply blood to the placental intervillous space. While this paternally controlled trophoblast invasion seeks to increase placental perfusion—and as a result, infant birth weight—the mother seeks to limit fetal growth so as not to die in childbirth. This conflict is fought in the human decidua where the mother’s immune surveillance attempts to limit trophoblast invasion. We have discovered a novel trophoblast secreted master immune regulator—PP13 (galectin 13)—which diverts the maternal immune surveillance away from the spiral arterioles, allowing trophoblast invasion, vascular conversion, and increased placental perfusion to proceed. The end result of this PP13 mediated diversion is a larger baby, with a concomitant larger brain, and increased chance of survival.