Plasma Osmolality and Urinary Concentration and Dilution During and After Pregnancy: Evidence That Lateral Recumbency Inhibits Maximal Urinary Concentrating Ability

Abstract

Summary In nine women studied serially before conception and through the first trimester, plasma osmolality (Posm) started to decline with the first missed menstrual period, was significantly decreased by the fifth week of pregnancy and was 10 mosmol/kg lower than preconception values by the tenth week, changing little thereafter. Changes in plasma sodium (and its attendant anion) accounted for the majority of the decrement. In separate studies, urinary concentration and dilution, assessed by water deprivation and loading, were studied in nine women during their last trimester and again 10–12 weeks post partum. Basal Posm was 9 mosmol/kg lower in the last trimester than post partum (p < 0.001) but the results of concentration and dilution tests were similar during and after pregnancy. Basal urinary arginine vasopressin (AVP) excretion was similar during and after pregnancy and water loading suppressed AVP excretion in both pregnant and postpartum women. An unanticipated observation was that lateral recumbency interfered with urinary concentration tests in both pregnant and postpartum women. The results demonstrate that the decrement in Posm during pregnancy is an early event. The data (urinary AVP excretion, its suppression by water loading and normal concentration and dilution despite a lower plasma tonicity) also suggest that human pregnancy is accompanied by a resetting of the threshold for vasopressin secretion to a lower Posm

Vasopressin and oxytocin levels during normal pregnancy: effects of chronic dietary sodium restriction

“…Theor etically, reduced circulating AVP levels might be antici pated during salt restriction. As reported by Davison et al (1981), decreased solute diuresis, as in restricted-salt diets, lowers distal tubular flow. Therefore, the fraction of solute escaping reabsorption and the residual volume to be concen trated are decreased, requiring decreased tubular binding of AVP.…”

“…Perhaps, in our study, diminished AVP plasma levels might have escaped detection because of assay sensitivity (see Introduction) or because patients in our study were positioned in the left lateral tilt position, neces sary for simultaneous Doppler flow measurements of the fetal and uterine circulation. In this position, the maximal concentrating ability of the kidney is diminished probably because of intravascular mobilized interstitial fluids which will consequently suppress AVP release (Davison et al 1981). Possible differences in neurophysin or AVP levels due to dietary restrictions might therefore have been blunted.…”

Pregnancy and pituitary disorders

“…The decrease starts by the first missed menstrual period and gradually decreases until the tenth week of gestation, which does not change later on (71). Serum sodium is decreased by about 4-5 mEq/ml due to this reset osmostat.…”

Reference values for clinical chemistry tests during normal pregnancy

“…This decrease in osmolality is accompanied by parallel decreases in the main extracellular fluid electrolytes, mainly sodium and its attendant ion chloride, as well as urea. 3 Furthermore, this group demonstrated robustly that the explanation for the observed decreases was not due simply to haemodilution, a commonly held and accepted explanation for most of the observed physiological changes in normal pregnancy, but was due instead to the lowering of the osmotic thresholds for both vasopressin release and thirst to the observed lower osmolality values. 4 This explanation, they argued, was why normally pregnant women were able to continue to concentrate and dilute their urine normally, about their new lower osmolality levels instead of being in a state of continuous diuresis as one would have expected if it was simply due to haemodilution.…”

“…2 Davison et al 3 in work spanning well over a decade, has demonstrated unambiguously, changes in osmoregulation function in the three trimesters of normal pregnancy, which we now know are physiological, but would have been interpreted as pathological in the context of nonpregnant reference values.…”