Nursing care
Why does preeclampsia cause proteinuria? From placenta to leaky kidney filter
Written and reviewed by Hannah Nguyen, MSN, RN, CPN · 4 min read · Updated November 2026
Short answer
Preeclampsia causes proteinuria because a poorly perfused placenta releases sFlt-1, a decoy protein that traps VEGF. The kidney filter needs VEGF to keep its capillaries and podocytes intact, so when VEGF is soaked up, the filter swells, podocytes are injured and protein leaks into the urine. Delivery removes the placental source, which is why it resolves afterward.
Step by step: placenta, sFlt-1 and the glomerular filter
The problem starts in the placenta, not the kidney. MSD Manuals lists poorly developed spiral arterioles, placental ischemia and immune factors among the likely causes, and names increased soluble Flt-1 (sFlt-1) as one contributor to the widespread vasospasm of preeclampsia. sFlt-1 is a free-floating receptor that grabs vascular endothelial growth factor (VEGF) before it can reach blood vessel walls.
The glomerulus leans hard on VEGF. A 2014 Kidney International review notes that sFlt-1 can bind and neutralize VEGF, leaving less free VEGF to support the vessels, and calls glomerular endotheliosis (swollen capillary lining cells) the classic kidney lesion of preeclampsia. It also describes altered podocyte proteins, such as nephrin and synaptopodin, as the point where these pathways converge. Podocytes are the cells wrapped around the glomerular capillaries, and once they are injured the filter leaks albumin into the urine.
Protein in the urine is a marker, not the main danger
A common student error is to treat the lost protein itself as the threat, as if it explains seizures or high blood pressure. It does not. The same blood vessel injury is happening in the brain, liver and placenta; the kidney is simply the organ where the damage is easiest to measure with a urine sample.
The review also notes that podocytes can show up in urine before the clinical features of preeclampsia appear, so kidney injury may begin earlier than the numbers suggest. The headache and visual changes that signal worsening disease come from the same vascular process, which the linked headache page explains, so do not read a stable protein result as a stable patient.
What protein level in urine means preeclampsia
MSD Manuals defines significant proteinuria in preeclampsia as more than 300 mg in 24 hours, a protein to creatinine ratio of 0.3 or higher, or a dipstick reading of 2+. These findings count only when new hypertension appears after 20 weeks of gestation; most cases develop after 34 weeks.
The dipstick is a screening tool with real limits. MSD Manuals states plainly that a negative result on less accurate tests, such as a dipstick or routine urinalysis, does not rule out preeclampsia. For the nurse, that means a trace or negative strip in a hypertensive pregnant client is reported along with the blood pressure, and the timed or ratio sample the prescriber orders is collected carefully rather than skipped.
Why preeclampsia causes swelling, and which swelling counts
Damaged vessel lining leaks fluid into the tissues, so edema and rapid weight gain often travel with the proteinuria. MSD Manuals flags sudden excessive gain of more than 5 lb in a week, and notes that nondependent edema, such as a swollen face or hands or a ring that no longer fits, is more specific than ankle swelling.
That distinction matters because swollen feet at the end of the day are common in ordinary late pregnancy. On assessment, weigh the client on the same scale at the same time, ask about rings and facial puffiness, and compare with earlier visits. A sudden jump in weight with puffy hands carries far more weight than lower leg swelling alone.
Hypothetical practice scenario
Hypothetical scenario, not a real exam item: a client at 35 weeks has a blood pressure of 150/96 on two readings and a urine dipstick showing trace protein. A student nurse says: "Her kidneys are fine, the strip is only trace," "I will ask whether her rings feel tight," and "I will check her weight against last week." Which statement needs correcting?
The first statement is the one to correct. A dipstick can miss significant proteinuria, and new hypertension after 20 weeks needs the ordered quantitative test and a report, not reassurance. The ring question and the weight comparison are sound, because nondependent edema and sudden gain point toward preeclampsia.
Sources and further reading
MSD Manual Professional Version: Preeclampsia and Eclampsia. Placental causes, sFlt-1 role in vasospasm, proteinuria thresholds, dipstick limits, onset after 20 weeks, edema and weight gain criteria.
Kidney International (2014): Advances in the pathophysiology of pre-eclampsia and related podocyte injury. sFlt-1 binding and neutralizing VEGF, glomerular endotheliosis as the classic lesion, altered podocyte proteins, podocyturia before clinical signs, rapid resolution of hypertension after placental removal.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our maternity and newborn practice questions are the closest set to what this page covers.
Common questions
What protein level in urine means preeclampsia?
MSD Manuals uses more than 300 mg of protein in 24 hours, a protein to creatinine ratio of 0.3 or higher, or a 2+ dipstick, together with new hypertension after 20 weeks. A negative dipstick does not rule preeclampsia out.
Why does preeclampsia cause swelling?
The same injury to blood vessel lining that opens the kidney filter lets fluid leak into tissues. Swelling of the face or hands and a gain of more than 5 lb in a week are more telling than ankle swelling, which is common in normal pregnancy.
Does the proteinuria go away after delivery?
It usually settles, because delivering the placenta removes the main source of sFlt-1 that drives the kidney injury. Blood pressure and urine findings are still monitored after birth, since preeclampsia can persist or worsen in the postpartum period under local protocol.