Brain natriuretic peptide (BNP) is synthesized by human fetal membranes, both the amnion and chorion. This locally produced BNP inhibits the contraction of the human myometrium, contributing to the maintenance of myometrial quiescence during pregnancy. We tested the hypothesis that BNP production is increased by fetal membrane stretching, which is predicted to occur in the expanding uterus, and inhibited by epidermal growth factor (EGF), whose production in the fetal membranes increases in late pregnancy. Term fetal membranes were obtained during elective cesarean delivery before labor. Sections of membranes were placed in an isolated chamber containing DMEM: F12 medium (37°C) and stretched with a 35 g weight. Medium and tissue samples were collected at 0, 3, 6, 18, and 24 hours for measurement of messenger RNA (mRNA) and BNP levels in the presence/absence of EGF (2 × 10(-9 )mol/L). Inducible nitric oxide synthase (iNOS) and β-actin were also evaluated to discard a nonspecific effect of mechanical stretch on protein expression. We found that amnion and chorion stretching increased the BNP mRNA (reverse transcription-polymerase chain reaction [RT-PCR]) and protein (radioimmunosorbent assay [RIA]) levels from 18 hours onward. The effect of stretching was inhibited by EGF (2 × 10(-9) mol/L). Stretch did not increase iNOS or β-actin protein levels. We concluded that chorion and amnion stretching may increase BNP expression in the fetal membranes during pregnancy, while increasing biological activity of EGF may decrease BNP production in the chorion and amnion late in pregnancy. We postulate BNP is an important regulator of myometrial contractility during pregnancy, and its production is modulated by both stretch and progressive increase in EGF levels during pregnancy.