Regulation of amniotic fluid volume: insights derived from amniotic fluid volume function curves

Abstract

Recent advances in understanding the regulation of amniotic fluid volume (AFV) include that AFV is determined primarily by the rate of intramembranous absorption (IMA) of amniotic fluid across the amnion and into fetal blood. In turn, IMA rate is dependent on the concentrations of yet-to-be identified stimulator(s) and inhibitor(s) that are present in amniotic fluid. To put these concepts in perspective, this review 1) discusses the evolution of discoveries that form the current basis for understanding the regulation of AFV, 2) reviews the contribution of IMA to this regulation, and 3) interprets experimentally induced shifts in AFV function curves and amnioinfusion function curves in terms of the activity of the amniotic fluid stimulator and inhibitor of IMA. In the early 1980s, it was not known whether AFV was regulated. However, by the late 1980s, IMA was discovered to be a "missing link" in understanding the regulation of AFV. Over the next 25 years the concept of IMA evolved from being a passive process to being an active, unidirectional transport of amniotic fluid water and solutes by vesicles within the amnion. In the 2010s, it was demonstrated that a renally derived stimulator and a fetal membrane-derived inhibitor are present in amniotic fluid that regulate IMA rate and hence are the primary determinants of AFV. Furthermore, AFV function curves and amnioinfusion function curves provide new insights into the relative efficacy of the stimulator and inhibitor of IMA.

Antimicrobial Activity of Human Fetal Membranes: From Biological Function to Clinical Use

“…In the third trimester, the hAF volume is maintained mainly by regulating the rate of absorption through the amnion into fetal blood. In details, it is controlled based on the ratio of fetal urine component that acts as a stimulator for absorption and a fetal membrane-derived inhibitor (Anderson et al, 2013;Brace et al, 2013Brace et al, , 2018. Secondly, the membranes must also withstand sudden impacts and compresses, for instance from fetal movements and Braxton-Hicks contractions.…”

Intrapartum Results on Differing Degrees of Ketonuria in Nulliparous Women with Gestational Diabetes Mellitus during Spontaneous Labor

“…Long-standing ketonuria is considered to be a surrogate measure of both dehydration and caloric balance [28], and maternal ketonemia that results in ketonuria is a hyperosmolar condition [29]. Studies on sheep showed that maternal conditions involving hyperosmolarity (such as maternal dehydration) resulted in a diminution of fetal amniotic fluid volume from decreased fetal urination [30], potentially due to attenuated fetal production of atrial natriuretic factor [31]. Similarly, several human studies showed that maternal hydration resulted in a significant increase in amniotic fluid volume [32].…”

Altered proteomics profile in the amnion of patients with oligohydramnios

“…Experimental studies in animal models led to the current concept that amniotic fluid is a dynamic reservoir maintained within the normal range through regulation of inflow and outflow pathways (Brace, Anderson, & Cheung, , ). At late gestation, fetal urine is the primary source of amniotic fluid while fetal swallowing removes fluid from the amniotic compartment (Brace et al, ). The intramembranous (IM) pathway allows transfer of fluid and dissolved solutes through the placental amnion from the amniotic cavity into the underlying fetal blood vessels on the surface of the placenta.…”

“…This is a unidirectional pathway in that the majority of fluid flow is outward from the amniotic compartment while inward movement is negligible Cheung & Brace, 2008). Previous research indicated that this IM pathway is the primary determinant of AFV (Brace et al, 2018). Furthermore, the placental amnion rather than the chorion determines the rate of IM transport and, as such, is the rate-limiting layer and site of AFV regulation (Adams, Choi, Cheung, & Brace, 2005).…”