The placenta forms from the trophectoderm of the blastocyst. The demethylation of the sperm and ovum DNA allows the de-repression of endogenous viral DNA elements and the transcription of the viral coat proteins syncytin 1 and 2. The syncytins cause the fusion of cytotrophoblast cells to form the key hormone producing layer of syncytiotrophoblast. Unlike other endocrine organs the syncytiotrophoblast secretes its steroid and peptide hormones continuously rather than in response to acute stimuli. As the placenta follows its maturational trajectory its production of hormones changes. Placental hormones act to facilitate implantation and to maintain pregnancy, to mobilize maternal nutrients to enable fetal growth, to promote the growth of the uterus, to stimulate breast development and lactogenesis, to orchestrate the onset of labor, and likely to alter maternal behavior before and after delivery to enhance the survival prospects of the fetus.