We aim to demonstrate that Brain Natriuretic Peptide (BNP) is synthesized and released from the fetal membranes and mediates pregnancy myometrial quiescence. Myometrium and fetal membranes (FM) were obtained from term and preterm pregnancies at the time of cesarean section, either in labor or not in labor. BNP was measured in term and preterm FM, in culture cells, and conditioned media. We found BNP (but not ANP or CNP) inhibited contractions of preterm, but not term, human myometrium. BNP (both protein and mRNA) was detected in all tissues, conditioned media and cultured cells. BNP was higher in samples from preterm women not in labor compared to those at term not in labor. BNP concentrations were significantly reduced in women in spontaneous preterm labor. We conclude that locally produced BNP may be involved in generating myometrial quiescence during pregnancy. Further, a premature decrease of BNP production may cause preterm labor.