The placenta plays a critical role in providing an environment that supports optimal fetal growth. It does this by providing the site of nutrient transfer from the mother to the fetus and waste secretion from the fetus to the mother, acting as a barrier against pathogens and the maternal immune system, and as an active endocrine organ capable of secreting hormones, growth factors, cytokines, and other bioactive products. Among the hormones produced by the placenta are members of the growth hormone/prolactin gene family, the placental lactogens (PL) and prolactin-related proteins. Although the exact functions of the placental members of this gene family have not been entirely elucidated, the available evidence supports a role for some in modulating maternal and fetal metabolism. The PL are secreted into the maternal and fetal circulations and, at least in ruminants, seem to mediate their effects through unique receptors, although this remains controversial. One action of the PL may be to modulate fetal IGF production. Research with mice, using gene ablation techniques, indicates the importance of the IGF for maintaining normal fetal growth rate. This research provided data on the timing of the onset of IGF effects on fetal growth and the receptors through which these effects are mediated. This review is about the structure, mechanism of action, and potential function of the placental members of the growth hormone/prolactin gene family and the recent evidence on the role of IGF in fetal growth regulation.