Oedema-preventing mechanisms in subcutaneous tissue of normal pregnant women.

Fluid transport between the plasma and interstitial fluid compartment is governed by the Starling forces, i.e. the capillary pressure (Pc), interstitial fluid hydrostatic pressure (Pi) and colloid osmotic pressure in plasma (COPp) and interstitial fluid (COPi). The COPp, COPi and Pi 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 Pi was measured by the 'wick-in-needle' technique. The COPp was reduced from 23.2 mmHg in the first trimester to 21.1 mmHg in the third trimester. Concomitantly, COPi 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 Pi were observed. The more marked fall in COPi than in COPp indicates that a rise in Pc, in addition to hypoproteinaemia, contributes to increased capillary fluid filtration in pregnancy. The reduction in COPi 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.