Residency · Residency · Nephrology

Pregnancy and the Kidney

Introduction

Pregnancy induces profound physiological changes in renal hemodynamics and function that are essential for supporting fetal development but also create unique clinical challenges. Pre-existing chronic kidney disease complicates 3 to 4 percent of pregnancies and significantly increases the risk of adverse maternal and fetal outcomes. Hypertensive disorders of pregnancy affect 5 to 10 percent of all pregnancies and are a leading cause of maternal and neonatal morbidity and mortality worldwide. One of the most critical diagnostic challenges in nephrology is distinguishing pre-eclampsia from an exacerbation of underlying CKD or other pregnancy-related renal diseases, a distinction that has direct implications for the timing of delivery and the need for disease-specific therapies.

Renal Physiology in Normal Pregnancy

Hemodynamic Changes

The cardiovascular system undergoes dramatic adaptation during pregnancy. Cardiac output increases by 30 to 50 percent by mid-pregnancy, driven by increases in both heart rate and stroke volume. Systemic vascular resistance decreases substantially through the vasodilatory effects of nitric oxide, relaxin, progesterone, and prostacyclin. As a consequence of this vasodilation, blood pressure normally decreases during the first and second trimesters, reaching a nadir at 20 to 24 weeks of gestation, before gradually returning toward pre-pregnancy levels by term. Plasma volume expands by 40 to 50 percent by 32 weeks, a proportionally greater increase than the accompanying expansion of red blood cell mass, which produces the physiologic anemia of pregnancy with a normal hemoglobin of approximately 11 g/dL.

Renal Hemodynamic Changes

Renal plasma flow increases by 50 to 80 percent by mid-pregnancy, and the glomerular filtration rate rises by approximately 50 percent, from a baseline of roughly 120 mL/min to 170 to 180 mL/min, by the end of the first trimester. This gestational hyperfiltration has critical implications for laboratory interpretation. The normal serum creatinine in pregnancy is 0.4 to 0.6 mg/dL, and a value of 0.8 mg/dL or higher, which would be considered normal in a non-pregnant individual, may indicate significant renal impairment in a pregnant woman. Blood urea nitrogen similarly decreases to 8 to 10 mg/dL. Serum uric acid decreases initially due to increased renal clearance but characteristically rises during the third trimester as clearance declines; a rising uric acid level in the third trimester may serve as an early marker of developing pre-eclampsia.

The kidneys increase in length by 1 to 1.5 cm during pregnancy, and physiologic hydronephrosis develops, more prominent on the right side due to dextrorotation of the enlarging uterus and compression of the right ureter by the right iliac vessels. This right-sided predominance is an important consideration when evaluating pregnant patients with flank pain or suspected obstruction.

Tubular and Hormonal Changes

Several tubular transport changes occur during normal pregnancy. Mild proteinuria of up to 300 mg per day is considered physiologic, resulting from the increased GFR and a modest increase in glomerular permeability. Glycosuria is common and results from the increased filtered glucose load exceeding the reduced tubular maximum for glucose reabsorption; importantly, glycosuria in pregnancy is not diagnostic of gestational diabetes and should not be used as a screening tool. Hypercalciuria develops from the combination of increased filtered calcium from the elevated GFR and increased intestinal calcium absorption driven by placental production of 1-alpha-hydroxylase, which converts 25-hydroxyvitamin D to its active 1,25-dihydroxyvitamin D form. Urinary calcium excretion approximately doubles during pregnancy.

Progesterone-mediated stimulation of the respiratory center produces a mild respiratory alkalosis, with PaCO2 decreasing to 30 to 32 mmHg. The kidneys compensate by increasing bicarbonate excretion, resulting in a normal serum bicarbonate of 18 to 20 mEq/L during pregnancy. Plasma osmolality decreases by approximately 10 mOsm/kg due to a reset osmostat, in which the threshold for ADH release is lowered, resulting in a serum sodium approximately 4 to 5 mEq/L lower than the non-pregnant state. This resetting is mediated by the placental hormone relaxin and by elevated human chorionic gonadotropin.

<image>Comprehensive diagram showing renal physiological changes in normal pregnancy. Show a timeline from pre-pregnancy through the three trimesters and postpartum. Plot the following parameters across the timeline: (1) GFR rising 50% by end of first trimester and sustained through delivery, then normalizing postpartum. (2) Serum creatinine declining to 0.4-0.6 mg/dL. (3) Blood pressure dipping in first-second trimester then rising to pre-pregnancy levels. (4) Plasma volume expanding by 40-50%. (5) Serum sodium declining by 4-5 mEq/L (reset osmostat). (6) Serum uric acid declining early then rising in third trimester. (7) Proteinuria threshold increasing to 300 mg/day. Include a side panel showing the drivers of renal vasodilation: nitric oxide, relaxin, progesterone, prostacyclin, and the net effect on afferent arteriolar tone. Show physiologic hydronephrosis with the right kidney more affected than the left.</image>

Hypertensive Disorders of Pregnancy

Classification (ACOG/ISSHP)

The classification of hypertensive disorders of pregnancy encompasses several distinct entities. Chronic hypertension is defined as blood pressure at or above 140/90 mmHg documented before 20 weeks of gestation or known to predate the pregnancy. Gestational hypertension is new-onset hypertension at or above 140/90 mmHg after 20 weeks without proteinuria or evidence of end-organ damage. Pre-eclampsia is defined as new-onset hypertension after 20 weeks accompanied by either proteinuria (at least 300 mg per 24 hours or a urine protein-to-creatinine ratio of 0.3 or greater) or evidence of end-organ damage, including thrombocytopenia below 100,000, renal insufficiency (creatinine above 1.1 mg/dL or a doubling of the baseline value), elevated hepatic transaminases above twice the upper limit of normal, pulmonary edema, or new-onset cerebral or visual symptoms. Pre-eclampsia with severe features is diagnosed when any of the following are present: systolic blood pressure at or above 160 mmHg or diastolic at or above 110 mmHg, thrombocytopenia below 100,000, renal insufficiency, hepatic transaminase elevation above twice normal, pulmonary edema, or cerebral or visual symptoms. Eclampsia denotes pre-eclampsia complicated by new-onset seizures. HELLP syndrome, a particularly severe manifestation, is characterized by the triad of hemolysis (schistocytes on peripheral smear, elevated lactate dehydrogenase, low haptoglobin), elevated liver enzymes, and low platelets. Chronic hypertension with superimposed pre-eclampsia describes the development of worsening hypertension, new or worsening proteinuria, or new end-organ damage after 20 weeks in a patient with pre-existing chronic hypertension.

Pre-Eclampsia Pathophysiology

The pathogenesis of pre-eclampsia is understood through a two-stage model. In the first stage, abnormal placentation occurs when cytotrophoblast cells fail to adequately invade and remodel the maternal spiral arteries during the first trimester. This results in retained muscular walls in the spiral arteries, preventing the normal conversion from high-resistance to low-resistance vessels, and produces chronic placental ischemia and hypoxia. In the second stage, the ischemic placenta releases anti-angiogenic factors into the maternal circulation, primarily soluble fms-like tyrosine kinase 1 (sFlt-1) and soluble endoglin, which sequester the pro-angiogenic factors VEGF and placental growth factor (PlGF). The resulting anti-angiogenic imbalance causes systemic maternal endothelial dysfunction, manifesting as hypertension, proteinuria, and multi-organ damage.

The sFlt-1 to PlGF ratio has emerged as a powerful clinical biomarker. A ratio below 38 effectively rules out the development of pre-eclampsia within 1 to 4 weeks with high negative predictive value. A ratio above 85 in early-onset disease or above 110 in late-onset disease has high positive predictive value for pre-eclampsia. The sFlt-1/PlGF ratio has received FDA approval for clinical use and is increasingly employed as a diagnostic aid in ambiguous presentations where the distinction between pre-eclampsia and other causes of hypertension and proteinuria in pregnancy is unclear. The pathognomonic renal lesion of pre-eclampsia is glomerular endotheliosis, in which endothelial cells swell and occlude the capillary lumen, producing characteristic "bloodless glomeruli" on biopsy.

Pre-Eclampsia Management

Delivery is the only definitive cure for pre-eclampsia, as the disease resolves with removal of the placenta. The timing of delivery is determined by the severity of disease and gestational age. Pre-eclampsia with severe features warrants delivery at 34 weeks or beyond, or immediately at any gestational age if maternal instability precludes further expectant management. Pre-eclampsia without severe features may be managed expectantly with close monitoring until 37 weeks. When preterm delivery is anticipated before 34 weeks, antenatal corticosteroids (betamethasone) should be administered to promote fetal lung maturity.

Antihypertensive therapy for acute severe hypertension (systolic at or above 160 mmHg or diastolic at or above 110 mmHg) employs intravenous labetalol (initial dose 20 mg, escalated to 40 mg and then 80 mg at 10- to 20-minute intervals), intravenous hydralazine (5 to 10 mg every 20 minutes), or oral nifedipine (10 to 20 mg every 20 to 30 minutes). For maintenance blood pressure control, labetalol, nifedipine extended-release, and methyldopa are the preferred agents, with a target below 140/90 mmHg. ACE inhibitors and angiotensin receptor blockers are absolutely contraindicated in pregnancy due to their teratogenic effects, which include renal agenesis, oligohydramnios, pulmonary hypoplasia, and neonatal renal failure. Sodium nitroprusside is also contraindicated because of the risk of fetal cyanide toxicity.

Seizure prophylaxis with magnesium sulfate is the standard of care. A loading dose of 4 to 6 grams is administered intravenously, followed by a maintenance infusion of 1 to 2 grams per hour, continued for 24 to 48 hours postpartum. Monitoring during magnesium therapy requires assessment of deep tendon reflexes (loss of reflexes occurs at serum levels of 8 to 12 mEq/L), respiratory rate (must remain above 12 per minute), urine output (must remain above 25 mL per hour to ensure renal clearance), and serum magnesium levels targeting 4 to 7 mEq/L. Magnesium toxicity progresses from loss of reflexes to respiratory depression (at levels of 12 to 18 mEq/L) to cardiac arrest (above 20 mEq/L); the antidote is calcium gluconate 1 gram administered intravenously.

Low-dose aspirin prophylaxis at 81 to 162 mg daily, initiated at 12 to 16 weeks of gestation, is recommended for women at high risk of pre-eclampsia. The ASPRE trial demonstrated a 62 percent reduction in preterm pre-eclampsia with aspirin prophylaxis. High-risk indications include prior pre-eclampsia, chronic hypertension, pregestational diabetes, CKD, systemic lupus erythematosus, and antiphospholipid syndrome.

Distinguishing Pre-Eclampsia from CKD Flare

CKD and Pregnancy

Pre-Conception Counseling

All women of reproductive age with CKD should receive comprehensive pre-conception counseling that addresses the risks to both mother and fetus and the medication adjustments required before conception. Risk stratification is based on CKD stage, degree of proteinuria, blood pressure control, and activity of the underlying disease. Women at low risk, with eGFR above 60, proteinuria below 1 gram per day, well-controlled blood pressure, and inactive disease, can generally expect good maternal and fetal outcomes with appropriate monitoring. Those at moderate risk, with eGFR between 30 and 59 and proteinuria of 1 to 3 grams per day, face increased rates of pre-eclampsia, preterm delivery, and accelerated CKD progression. Women at high risk, with eGFR below 30 or proteinuria exceeding 3 grams per day or uncontrolled hypertension, face substantial maternal risk including pre-eclampsia in 40 to 60 percent, preterm delivery in 70 to 90 percent, accelerated CKD progression with potential need for dialysis, and significant fetal risk including growth restriction and stillbirth.

Medication Adjustments

Medication review and adjustment is a critical component of pre-conception planning. Medications that should be continued include labetalol, nifedipine, and methyldopa as safe antihypertensive agents; low-dose aspirin if indicated for pre-eclampsia prophylaxis; hydroxychloroquine, which is essential for disease control in SLE and has an established safety profile in pregnancy; azathioprine, which is the preferred antimetabolite in pregnancy; and tacrolimus, though levels must be monitored closely as pregnancy-related increases in metabolism may necessitate dose increases.

Medications that must be discontinued include ACE inhibitors and ARBs, which are teratogenic in all trimesters and should be switched to safe alternatives before conception or immediately upon pregnancy confirmation. Mycophenolate mofetil is teratogenic, causing characteristic facial clefts and ear and limb defects, and must be switched to azathioprine at least 3 months before planned conception to allow adequate washout. Methotrexate, cyclophosphamide, statins, and warfarin (which is teratogenic in the first trimester) must also be discontinued. SGLT2 inhibitors should be discontinued in pregnancy based on limited human safety data and concerning animal studies.

Dialysis in Pregnancy

Pregnancy is possible in women receiving hemodialysis, though it remains rare and carries high risk. Intensive hemodialysis, defined as at least 36 hours per week delivered across 5 to 6 sessions, dramatically improves pregnancy outcomes compared to conventional thrice-weekly hemodialysis. The live birth rate improves from approximately 40 to 50 percent with conventional schedules to approximately 85 percent with intensive hemodialysis, with better fetal growth, fewer preterm deliveries, and improved maternal metabolic control. The rationale for intensive dialysis is to maintain the blood urea nitrogen below 45 to 50 mg/dL, because urea freely crosses the placenta and elevated fetal urea levels produce osmotic diuresis and polyhydramnios. Additional management considerations include addressing the higher erythropoietin-stimulating agent requirements to maintain a hemoglobin target of 10 to 11 g/dL, maintaining calcium and phosphorus homeostasis, and meticulously avoiding intradialytic hypotension, which compromises uteroplacental perfusion.

Transplant and Pregnancy

Kidney transplant recipients are advised to wait at least 1 year post-transplant before attempting pregnancy, provided graft function is stable with eGFR above 40, proteinuria is minimal, and there has been no recent rejection episode. Safe immunosuppressive medications during pregnancy include tacrolimus (with anticipation of the need for dose increases as pregnancy-related hepatic metabolism increases), azathioprine, and low-dose prednisone. Mycophenolate must be switched to azathioprine at least 3 months before conception, and mTOR inhibitors should be avoided due to limited safety data.

Pregnancy outcomes in transplant recipients with well-functioning grafts are generally favorable, with a 90 to 95 percent live birth rate. However, the risks of pre-eclampsia (approximately 30 percent), preterm delivery (45 to 50 percent), and small-for-gestational-age infants remain elevated compared to the general population. There is no proven increase in acute rejection during pregnancy when immunosuppression is maintained at stable levels. Breastfeeding is compatible with prednisone, azathioprine, and tacrolimus, all of which have minimal excretion in breast milk.

<image>Clinical management timeline for pregnancy in a woman with CKD. Show a horizontal timeline from pre-conception through pregnancy trimesters to postpartum. Pre-conception phase: medication review (switch ACEi/ARB to labetalol/nifedipine, switch MMF to azathioprine 3 months before, continue HCQ), disease optimization (wait for remission, achieve BP <140/90), start folate, genetic counseling if Alport/PKD. First trimester: confirm viable pregnancy, baseline labs (creatinine, UPCR, CBC, liver function), start low-dose aspirin 81 mg at 12-16 weeks if high-risk. Second trimester: monitor BP and proteinuria every 2-4 weeks, serial fetal growth US, monitor creatinine (remember pregnancy normal is 0.4-0.6). Third trimester: intensify monitoring (weekly BP, biweekly labs), watch for pre-eclampsia (sFlt-1/PlGF ratio if ambiguous), antenatal steroids if preterm delivery anticipated. Delivery and postpartum: restart ACEi/ARB if breastfeeding compatible (enalapril, captopril safe), monitor for postpartum pre-eclampsia (can present up to 6 weeks postpartum), long-term follow-up for CKD progression and future pregnancy counseling.</image>

Pregnancy-Specific Renal Diseases

Acute Fatty Liver of Pregnancy (AFLP)

Acute fatty liver of pregnancy is a rare but life-threatening condition that occurs in the third trimester, characterized by microvesicular hepatic steatosis leading to acute liver failure. The presentation includes elevated hepatic transaminases, coagulopathy, hypoglycemia, and AKI. AFLP can overlap clinically with HELLP syndrome, but distinguishing features include hypoglycemia (which is uncommon in HELLP), prolonged prothrombin time and INR, and low fibrinogen levels reflecting true hepatic synthetic failure. An important genetic association exists with long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency in the fetus, a fatty acid oxidation disorder that impairs placental and maternal metabolism. Treatment is emergent delivery with supportive care for liver and renal failure; liver transplantation is rarely required.

Thrombotic Microangiopathies in Pregnancy

The thrombotic microangiopathies of pregnancy pose particularly challenging diagnostic dilemmas. HELLP syndrome, the most common pregnancy-associated TMA, is a variant of severe pre-eclampsia and resolves within 72 hours of delivery. Thrombotic thrombocytopenic purpura may present or relapse during pregnancy, with ADAMTS13 activity below 10 percent serving as the diagnostic criterion; treatment requires plasma exchange with the addition of caplacizumab. Complement-mediated atypical hemolytic uremic syndrome characteristically presents in the postpartum period and, critically, does not resolve with delivery, distinguishing it from HELLP. Treatment requires complement-directed therapy with eculizumab, and renal biopsy may be needed when the diagnosis is uncertain. The key distinguishing features are temporal: HELLP resolves within 72 hours of delivery, TTP has severely reduced ADAMTS13 activity, and aHUS presents postpartum and persists despite delivery.

Gestational Diabetes Insipidus

Gestational diabetes insipidus is caused by excessive placental production of vasopressinase (cysteine aminopeptidase), an enzyme that degrades endogenous arginine vasopressin. The condition typically presents in the third trimester with polyuria and polydipsia, and serum sodium may be elevated or normal depending on the patient's ability to match increased water intake to the increased losses. Treatment is with desmopressin (DDAVP), which is resistant to vasopressinase degradation due to its structural modification at the first amino acid position. The condition resolves within 1 to 2 weeks postpartum as the placenta is delivered and vasopressinase levels decline. Risk factors include multiple gestations, which increase the total placental mass and vasopressinase production, and pre-eclampsia or HELLP syndrome, in which impaired hepatic clearance of vasopressinase elevates its circulating levels.

Key Clinical Pearls

• Serum creatinine >=0.8 mg/dL in pregnancy may indicate significant renal impairment; normal pregnant creatinine is 0.4-0.6 mg/dL due to 50% GFR increase
• The sFlt-1/PlGF ratio is the most useful biomarker for distinguishing pre-eclampsia from CKD flare or chronic hypertension; a low ratio (<38) has high negative predictive value for pre-eclampsia development within 1-4 weeks
• ACEi/ARBs are contraindicated in pregnancy (teratogenic); switch to labetalol, nifedipine, or methyldopa BEFORE conception; MMF must be switched to azathioprine at least 3 months before planned pregnancy
• Intensive hemodialysis (>=36 hours/week) dramatically improves pregnancy outcomes in dialysis patients; BUN target <45-50 mg/dL to prevent fetal complications
• Atypical HUS presenting postpartum does NOT resolve with delivery (unlike HELLP); complement-directed therapy with eculizumab is required; ADAMTS13 activity distinguishes TTP from aHUS

References

1. Wiles K, Chappell LC, Lightstone L, Bramham K. Updates in Diagnosis and Management of Preeclampsia in Women with CKD. Clin J Am Soc Nephrol. 2020;15(9):1371-1380.
2. Zeisler H, Llurba E, Chantraine F, et al. Predictive Value of the sFlt-1:PlGF Ratio in Women with Suspected Preeclampsia. N Engl J Med. 2016;374(1):13-22.
3. Hladunewich MA, Hou S, Odutayo A, et al. Intensive Hemodialysis Associates with Improved Pregnancy Outcomes: A Canadian and United States Cohort Comparison. J Am Soc Nephrol. 2014;25(5):1103-1109.
4. Piccoli GB, Cabiddu G, Attini R, et al. Risk of Adverse Pregnancy Outcomes in Women with CKD. J Am Soc Nephrol. 2015;26(8):2011-2022.
5. ACOG Practice Bulletin No. 222: Gestational Hypertension and Preeclampsia. Obstet Gynecol. 2020;135(6):e237-e260.

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