Oedema-preventing mechanisms in subcutaneous tissue of normal pregnant women

Summary. Fluid transport between the plasma and interstitial fluid compartment is governed by the Starling forces, i.e. the capillary pressure (P_c). interstitial fluid hydrostatic pressure (P_i) and colloid osmotic pressure in plasma ( COP _i) and interstitial fluid ( COP _i). The COP _P , COP _i and P _i were measured in 10 normal pregnant women in the first and 10 women in the third trimester of pregnancy. Interstitial fluid was collected from subcutaneous tissue by implanted wicks and P _i was measured by the'wick-in-needle'technique. The COP _P was reduced from 23.2 mmHg in the first trimester to 21.1 mmHg in the third trimester. Concomitantly, COP _i decreased from 13.1 to 8.4 mmHg on the thorax and from 9.6 to 5.5 mmHg at the ankle. Only small changes in P _i were observed. The more marked fall in COP _i than in COP_P indicates that a rise in P _e, in addition to hypoproteinaemia, contributes to increased capillary fluid filtration in pregnancy. The reduction in COP _i opposes the increased filtration and thereby prevents a rise in interstitial fluid volume and oedema formation. These physiological changes imply a reduced safety margin against oedema formation in late pregnancy.