Mechanical stretch increases brain natriuretic peptide production and secretion in the human fetal membranes

Normal amino-terminal pro-brain natriuretic peptide (NT-proBNP) values in amniotic fluid

INTRODUCTION

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. Reference values for NT-proBNP concentrations in amniotic fluid at different gestational ages have not been completely defined. We aimed to investigate the range of fetal NT-proBNP values in amniotic fluid in normal pregnancy between 17 and 41weeks of gestation.

METHODS

Samples of amniotic fluid were obtained from women meeting the following inclusion criteria: gestational age defined by early ultrasound, singleton gestation and not in labor. The exclusion criteria were as follows: multiple gestation, clinically evident chorioamnionitis, laboratory signs of infection in the amniotic fluid sample and fetal conditions that may alter NT Pro-BNP levels (anemia, hydrops, etc.). NT-proBNP concentrations in amniotic fluid were measured using the automated Elecsys® proBNP assay.

RESULTS

We analyzed 218 samples of amniotic fluid at various gestational ages. Half of the samples were obtained by amniocentesis (118 samples), and the other half (100 samples) were obtained by direct puncture at the time of cesarean section. We found a significant decline in NT-proBNP concentrations with advancing gestational age.

DISCUSSION

Gestational age has to be taken into consideration in the assessment of NT-proBNP values. Our data may be used as reference values in fetal medicine, as a possible predictor of preterm delivery risk using the inferior limit (0.5 multiples of the median (MoM)) of our normal curve.

The role of brain natriuretic peptide in maintaining myometrial quiescence during pregnancy

Myometrial quiescence is a physiological stage of the myometrium during pregnancy. It is a period of active relaxation of the myometrial smooth muscle cells; myometrial quiescence is responsible for maintaining pregnancy. The precise mechanisms underlying myometrial quiescence have not been completely elucidated, although many mediators and cellular pathways have been described as playing a role. Fetal membranes (chorion and amnion) produce and release one or more substances that inhibit myometrial contractions, playing a central role in the maintenance of myometrial quiescence. Brain natriuretic peptide (BNP) is more potent than any other natriuretic peptide in inhibiting myometrial contractions in vitro. Brain natriuretic peptide is produced by the chorion and amnion, mainly during myometrial quiescence, and decreasing towards the end of pregnancy. Production of BNP is reduced in fetal membranes obtained from women in preterm labour. It is postulated that BNP, acting in a paracrine fashion, plays a key role in the maintaining myometrial quiescence and, therefore, controlling the duration of pregnancy. Furthermore, it is postulated that a premature decrease of BNP production by the fetal membranes may cause preterm labour and preterm birth.