Brain natriuretic peptide (BNP) is synthesized by human fetal membranes. BNP contributes to the maintenance of myometrial quiescence during pregnancy. The preferred method of estimating BNP concentration is to measure the NT-proBNP concentration. We measured NT-proBNP values in amniotic fluid in normal pregnancy and built a normal curve to be used as reference values in fetal medicine. NT-proBNP concentrations declined with advancing gestational age.