In non-pregnant individuals, abnormal total protein excretion is typically defined as greater than 150 mg daily. In normal pregnancy, urinary protein excretion increases substantially, due to a combination of increased glomerular filtration rate and increased permeability of the glomerular basement membrane . Hence, total protein excretion is considered abnormal in pregnant women when it exceeds 300 mg/24 hours .
Proteinuria is one of the cardinal features of preeclampsia (table 1), which is a common and potentially severe complication of pregnancy. However, two important points should be noted. First, the severity of proteinuria is not indicative of the severity of preeclampsia and should not be used to guide management [3-5]. Second, although part of the formal diagnostic criteria of preeclampsia, proteinuria may be absent. Studies have shown that 10 percent of women with clinical and/or histological manifestations of preeclampsia have no proteinuria and 20 percent of women with eclampsia do not have significant proteinuria prior to their seizure [6,7].
Although less prevalent, primary renal disease and renal disease secondary to systemic disorders, such as diabetes or primary hypertension (formerly called “essential” hypertension), are also usually characterized by proteinuria and may first present in pregnancy. To further complicate this picture, 20 to 25 percent of women with chronic hypertension and diabetes develop superimposed preeclampsia [8,9].
It is important for clinicians caring for pregnant women to understand how to identify proteinuria, and how to determine whether preeclampsia or renal disease (or both) is the cause. This topic will discuss the approach to the evaluation of pregnant women with proteinuria and management of nephrotic syndrome in pregnancy. The evaluation of proteinuria in nonpregnant individuals and measurement of protein excretion are discussed in detail separately. (See "Assessment of urinary protein excretion and evaluation of isolated non-nephrotic proteinuria in adults".)
RENAL CHANGES IN NORMAL PREGNANCY
Glomerular filtration rate (GFR) and renal blood flow rise markedly during pregnancy, resulting in a physiologic fall in the serum creatinine concentration. As discussed above, urinary protein excretion increases substantially due to a combination of increased GFR and increased permeability of the glomerular basement membrane . Women with uncomplicated twin pregnancies have greater increases in urinary protein excretion than do women with singleton pregnancies . Additional information on pregnancy-related changes in renal function and the urinary tract can be found separately. (See "Renal and urinary tract physiology in normal pregnancy".)