Studies across several species, particularly the mouse, show that growth hormone (GH, somatotrophin) is an important determinant of litter size, and to a lesser extent, of birth length. GH acts at all stages of development, from ovulation through preimplantation development to the late fetus, with actions on both embryo/fetus and mother contributing to successful fetal development. The fact that these are not more obvious in vivo is likely a result of redundancy of cytokine hormone action, particularly in relation to prolactin, which shares common actions and receptor locations with GH.