ABSTRACT.Plasma and interstitial fluid colloid osmotic pressure (COP) and interstitial fluid hydrostatic pressure (Pi) were measured in 10 normal pregnant women in first and third trimester. Interstitial fluid COP was measured by a “wick” technique and Pi by a “wick‐in‐needle” technique. The COP was significantly lower both in plasma and interstitial fluid in the third trimester compared to that in the first trimester. However, the fall in COP was more marked in the interstitial fluid than in plasma. Only small changes in Pi were observed. Our results support the view that reduction of COP in interstitial fluid is a major local edema‐preventing mechanism. This compensates for the increased capillary pressure and the hypoproteinemia in late pregnancy. The relatively low interstitial fluid COP in third trimester women without edema implies a reduced safety margin against edema formation and may explain why 40 ‐ 80 % of normal pregnant women e