Pregnancy changes a lot of things in a mother: the body goes through many transformations trying to adapt to the needs of the baby. The mother’s behavior changes as well, as she has to care for her newborn. There is some indirect evidence that the baby (through its genes) could instruct the mother’s brain and alter her behavior even before it is born. But is this really true? We have previously found a gene that controls the number of hormones produced by the placenta, an organ that connects the developing embryo to the mother. To test if the embryos could affect the mother’s behavior, we conducted a series of experiments on mice who were pregnant with embryos expressing different doses of this gene. Indeed, mothers exposed to higher doses of the gene preferred nest-building over baby nursing.