1. The purpose of this study was to determine whether the 24 h urinary albumin excretory rate was increased in the third trimester of normal pregnancy or in pregnant women with hypertension who had 24 h urinary total protein excretion within the normal range.

2. Twenty-four hour urinary creatinine and albumin excretions were determined prospectively in 26 non-pregnant and 115 pregnant women in their third trimester (40 in normal pregnancy, 38 with mild pre-eclampsia, 20 with severe pre-eclampsia, 17 with essential hypertension) in whom urinary total protein excretion was normal. Both the urinary albumin excretion rate and the urinary albumin/creatinine ratio were compared among the groups. The clearance of albumin relative to that of creatinine was also calculated in the hypertensive women and in 14 of the non-pregnant women and nine of the normal pregnant women.

3. The twenty-four hour urinary albumin excretion rate was similar in non-pregnant [8(5–10) mg/day; median (interquartile range)] and normal pregnant [7(6–10) mg/day] women. Women with essential hypertension [6(4–16) mg/day] and mild pre-eclampsia [7(4–10) mg/day] had a urinary albumin excretion rate similar to that of normal pregnant women. Women with severe pre-eclampsia had an urinary albumin excretion rate increased [13(7–32) mg/day] compared with other groups (P < 0.05). The clearance of albumin relative to that of creatinine was elevated significantly only in women with severe pre-eclampsia compared with normal pregnant women (0.00054 versus 0.00012; P < 0.05). Blood pressures were similar among all hypertensive groups. The upper limit of the urinary albumin excretion rate was 20 mg/day in normal pregnant women, similar to that in non-pregnant women.

4. Urinary albumin excretion is unchanged in the third trimester of normal pregnancy, but is increased in women with features of severe pre-eclampsia who have normal total urinary protein excretion. This is not fully explained by a reduction in glomerular filtration rate nor by different systemic blood pressures, and probably reflects enhanced glomerular capillary permeability, possibly with altered maximum renal tubular reabsorption capacity for albumin.

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