Colloid osmotic pressure: Variations in normal pregnancy

Abstract

Pregnancy is associated with an increase in the extravascular fluid by about two litres. The regulation of fluid flux across capillary membrane is dependent on the STARLING's forces. Thus increased fluid shift into the extravascular space may occur during pregnancy when the intra-capillary hydrostatic pressure exceeds the opposing influence of plasma colloid oncotic pressure (COP). In order to evaluate these forces more fully, COP, hematocrit (HCT), serum total solids (STS), mean blood pressure (MBP) and MBP-COP gradient (M-C) were measured cross-sectionally in 184 women with normal uncomplicated pregnancy. COP fell gradually during the first and second trimesters, lowest at 30-34 weeks, thereafter it rose. The relationship of COP and gestational age was best described by the quadratic equation. The patterns of changes of HCT and STS with respect to gestational age were similar to COP. MBP and M-C were linearly correlated to gestational age. The increment in M-C with advancing gestational age would promote fluid flux from intra to extravascular space in normal pregnancy.