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ARTICLE
Pregnancy and the kidneys
N
Wearne, BA Med Sci (University of Sydney), MB ChB
Honours (University of Sydney), FCP (SA), Cert Nephrology (SA)
Division
of Nephrology and Hypertension, Department of Medicine,
Faculty of Health Sciences, Groote Schuur Hospital and
University of Cape Town, South Africa
Renal disease in pregnancy may cause a feeling of trepidation, even in the most experienced physician. However, before disease can be established, it is important to understand the substantial physiological changes that may occur during a normal pregnancy. Renal disease may take several forms and pregnancy may be the first medical review for women with a previously undiagnosed renal problem. Patients may have pre-existing renal disease, e.g. diabetic nephropathy. Additionally, women with renal transplants and renal diseases, e.g. lupus nephritis, require immunosuppression. Hypertensive disorders of pregnancy, including pre-eclampsia, are the commonest medical complications in pregnancy, and remain the most prevailing direct cause of maternal mortality in South Africa (SA). Both pre-existing hypertension and renal disease increase the risk of pre-eclampsia, which predisposes to preterm delivery, and maternal morbidity and mortality.
Pregnancy outcomes in renal
disease are determined by baseline creatinine levels,
hypertension and degree of proteinuria. The risk of progression
of chronic kidney disease increases as renal function worsens.
In SA, this is complicated by restricted access to dialysis in
the state sector. To ensure the best outcome for mother and
child, pre-pregnancy counselling and review of medication are
essential. Renal patients and those with hypertension are at
high risk of complications, and regular antenatal assessments by
a multidisciplinary team are required to monitor blood pressure,
proteinuria, diabetes control and fetal wellbeing.
S Afr Med J 2014;104(9):642.
DOI:10.7196/SAMJ.8765
Renal disease in pregnancy may cause a feeling of
trepidation, even in the most experienced
physician, and may take several forms. Pregnancy may be the
first medical review for women with a previously undiagnosed
renal problem. Patients may have pre-existing renal disease,
e.g. diabetic nephropathy. Additionally, women with renal
transplants and renal diseases, e.g. lupus nephritis, require
immunosuppression. Pregnancy outcomes in renal disease are
determined by baseline creatinine levels, hypertension and
degree of proteinuria. The risk of progression of chronic
kidney disease (CKD) and maternal and fetal complications
increases as the renal function worsens, and patients need to
be counselled, preferably prior to conception. Pre-existing
renal disease predisposes to pre-eclampsia, a condition
occurring in up to 5% of pregnancies, making it one of the
most common complications of pregnancy. A multidisciplinary
team approach involving a physician/nephrologist and an
obstetrician is essential to assess and care for pregnant
women with kidney disease.
Normal physiological adaptations of pregnancy
The kidneys increase in length (1 - 2 cm) and volume (70%) with dilation of the calyces, renal pelvis and ureters, features which are more prominent on the right. By the third trimester, 80% of women show evidence of hydronephrosis, which predisposes to urinary stasis, increasing the risk of urinary tract infections.1 There is hormonal dilatation of the systemic blood vessels, leading to a decrease in systemic vascular resistance and a physiological lowering of blood pressure, especially during the first and second trimesters. Thereafter, there is a steady return of blood pressure to pre-pregnancy levels until term.
Plasma volume increases significantly, leading to a 30 - 50% increase in cardiac output and resulting in a steady increase in renal blood flow (50% by the mid-second trimester). This increases the glomerular filtration rate (GFR) and gives rise to frequency and nocturia, resulting in decreased creatinine blood levels. The increased vascularity of the kidneys makes a renal biopsy beyond 32 weeks risky. Despite the increase in renal blood flow, the pressure within the glomerulus remains unaltered owing to compensatory effects on the afferent and efferent arteriole, but this only occurs in a normal kidney. Any pre-existing disease in the kidney will be accelerated, often with deterioration in renal function.
Pregnancy also affects the renal
tubules. There is decreased reabsorption of glucose and
bicarbonate, leading to glycosuria and bicarbonaturia. Despite
the loss of bicarbonate, there is a persistent alkalosis owing
to an increased respiratory rate. There is hypercalciuria,
although the risk of renal stones is not increased as inhibitors
of stone formation also increase. It is also common to find mild
proteinuria, <300 mg/d, which may remain up to 6 months
postpartum. Isolated pyuria is common and disappears by 3 months
postpartum. Serum sodium may decrease by 4 - 5 mEq/L and
serum osmolality falls to a new osmotic setpoint of about
270 mOsm/kg.
Important renal adaptations in pregnancy
• Increase in kidney size 1 - 2 cm
• Dilatation of the urinary collecting system
• Increase in renal blood flow
• Increase in glomerular filtration rate
• Decrease in creatinine
• Glycocosuria and bicarbonaturia
• Decrease in plasma sodium and osmolality
Estimating glomerular filtration rate in pregnancy
The modification of diet in renal disease (MDRD) is a
creatinine-based formula that measures GFR. It has become a
standard clinical method used to estimate renal function in
patients with CKD. The use of this formula has not been well
studied in the pregnant population and guidelines for its use
exclude pregnant women,2 because creatinine falls in
pregnancy owing to a pregnancy-induced increase in GFR and
haemodilution from plasma expansion. The MDRD formula is also
known to be inaccurate once the GFR is >60 ml/min/m2, even in
non-pregnant women. This inaccuracy is more pronounced in
pregnancy.3
Weight-based formulas, such as those used in the Cockroft-Gault
method, are likely to overestimate GFR because the increased
body weight in pregnancy does not typically reflect increased
muscle mass or creatinine production.4 Given this, 24-hour urine
collection for creatinine clearance remains the gold standard
for GFR estimation in pregnancy.
Estimating proteinuria in pregnancy
Proteinuria needs to be measured first to monitor pre-existing proteinuria, which may increase substantially during pregnancy. The protein-creatinine ratio (PCR) is the preferred method for quantitating proteinuria in the non-pregnant state5 and its use is recommended to monitor pre-existing proteinuria during pregnancy. The second indication is for the diagnosis of pre-eclampsia, which is defined as the new onset of hypertension (blood pressure (BP) >140/90 mmHg) and proteinuria (>300 mg protein in a 24-hour urine collection) after 20 weeks’ gestation.6 Although there are limitations to 24-hour urine collections, the use of PCR for the diagnosis of pre-eclampsia has been controversial, with discordant conclusions compared with 24-hour urine collections.7
Specific diseases of the kidney
Hypertensive disorders, including pre-eclampsia
Hypertensive disorders of pregnancy are the commonest medical complications in pregnancy, and remain the most prevailing direct cause of maternal mortality in SA.8 In the UK, 3 - 5% of pregnant women have hypertension that predates conception.9 It is more common in advanced maternal age, obesity and blacks.10 A secondary cause should be excluded if appropriate. About 22 - 25% of women with pre-existing hypertension are likely to develop pre-eclampsia.11 Pregnancy-induced hypertension is defined as a BP >140/90 mmHg arising after 20 weeks of gestation and returning to normal within 6 weeks postpartum.6 Pre-eclampsia is a multisystem disorder that is commonly associated with oedema and hyperuricaemia. It is a leading cause of maternal mortality and morbidity in developed12 and developing countries, where it continues to claim the lives of >60 000 mothers every year.13
Pre-eclampsia appears to be initiated by abnormal implantation of the placenta, leading to hypoperfusion and ischaemia. There is accumulating evidence supporting the release of several placental anti-angiogenic factors including soluble fms-like tyrosine kinase-19 (sFlt1) and its synergist, soluble endoglin (sEng). These anti-angiogenic factors are induced or exacerbated by placental ischaemia, with sFlt1 being a circulating antagonist to both vascular endothelial growth factor (VEGF) and placental growth factor. Circulating VEGF is mopped up by sFlt1to prevent the former’s protective effects on the endothelium. When administered to pregnant rats, sFlt1 and sEng produce a syndrome resembling that of HELLP (haemolysis, elevated liver enzymes, and low platelets).14 , 15 Subsequent studies have demonstrated the role of these anti-angiogenic factors in the pathogenesis of pre-eclampsia in humans.16-18
Clinically, vascular endothelial dysfunction and microangiopathy are present in the mother, but not in the fetus. The predominant target organ may be the brain (seizures or eclampsia), the liver (HELLP syndrome), or the kidney (glomerular endotheliosis and proteinuria). Predisposing factors include pre-existing hypertension, CKD, obesity, diabetes mellitus, thrombophilias, and multiple gestations.19 These women may be more susceptible to endothelial dysfunction and hence more responsive to anti-angiogenic factors.
Screening for pre-eclampsia has the greatest impact on clinical outcomes, but to date there is no effective preventive treatment. Although two large meta-analyses have suggested a small, but significant, overall benefit in patients treated with aspirin,20 , 21 the delivery of the fetus is currently the only definitive treatment.
Considerable evidence suggests that pre-eclampsia predisposes women to late cardiac and vascular diseases22 and also increases the risk of subsequent CKD. The risk is greater if a pre-eclamptic pregnancy results in the birth of a low-birth-weight or preterm infant or if pre-eclampsia occurs in more than one pregnancy.23
Chronic kidney disease in pregnancy
Women with CKD who become pregnant are at increased risk for
adverse maternal and fetal outcomes, including a rapid decline
in renal function, intrauterine grown retardation, perinatal
mortality and pre-eclampsia.24
,
25 The major determinants
of outcome are pre-existing proteinuria, hypertension, urinary
tract infection and degree of renal dysfunction. Obstetric
outcomes are also partly related to the level of available
neonatal care. Very importantly, maternal renal function may
decline irreversibly proportional to the level of pre-existing
dysfunction. In a SA setting, this is an important consideration
as access to dialysis is limited in the state sector.
Maternal renal function declines proportional to the level of pre-existing renal dysfunction 24 , 26
• Creatinine <123 µmol/L: considered safe
• Creatinine 130 - 170 µmol/L: 40% have deterioration in renal function, of whom half recover
• Creatinine >176 µmol/L: two-thirds have an accelerated decline in renal function and one-third require dialysis
The stress of increased renal blood flow and inability of the
glomerulus to regulate intraglomerular pressure, which normally
remains unaltered during pregnancy, may exacerbate renal damage
in pre-existing disease.
Managing dialysis and renal transplant patients
CKD is generally characterised by menstrual disturbances
and infertility, and is reversed by transplantation but not by
dialysis.27 Although conception
in patients on dialysis is unusual, it is not impossible, and
adequate contraception remains important. When pregnancy does
occur, it needs to be co-managed by a nephrologist and an
obstetrician as significant changes in management are required
to improve maternal and neonatal outcomes. Transplant patients
should also be co-managed as immunosuppressive drugs may need
to be altered and their plasma levels will need to be
carefully monitored. Pregnant women who have a transplanted
kidney with renal dysfunction should be considered in the same
way as other patients with CKD.
Pregnancy-induced acute kidney injury
Acute kidney injury (AKI) in pregnancy may be caused by any of
the disorders leading to renal failure in the general
population, e.g. acute tubular necrosis (ATN) due to infection,
glomerulonephritis or drug toxicity. There are, however,
pregnancy-related complications characteristic of each trimester
that can result in renal failure.28
,
29 Early in pregnancy the
most common problems are pre-renal disease due to hyperemesis
gravidarum or ATN following a septic abortion. In the second and
third trimesters, pregnancy-specific conditions, e.g.
pre-eclampsia, HELLP syndrome, acute fatty liver of pregnancy,
haemolytic-uraemic syndrome and thrombotic
thrombocytopenic purpura, can be complicated by acute renal
failure. At delivery, placental abruption can be associated with
severe ATN and bilateral cortical necrosis. The management of
AKI is similar to that in a non-pregnant patient, with fluid
management being especially important, particularly in the
context of pre-eclampsia, as the vascular endothelium is very
leaky. Potential nephrotoxins such as the non-steroidal
anti-inflammatory drugs (NSAIDs) and aminoglycosides should be
used with caution throughout the pregnancy.
Managing the complicated patient
Diabetic nephropathy
Diabetic nephropathy is frequently encountered during
pregnancy. Pre-pregnancy counselling is important, as
obstetric and renal outcomes are determined by the degree of
peripartum CKD. There is no increased risk of overt
nephrophathy or end-stage renal disease in women with normal
renal function at conception.30 However, diabetic
nephropathy is associated with a 2 - 4-fold increased risk of
pre-eclampsia, preterm delivery and perinatal death.31
Management includes low-dose aspirin from the first trimester
to reduce the risk of pre-eclampsia, regular renal function
monitoring, good BP control and tight glycaemic control.
Lupus nephritis
Women with lupus nephritis should be advised to conceive after a 6-month period of quiescent disease. Predictors of poor obstetric outcomes include active disease at conception and early pregnancy, >0.5 g protein/24 h, CKD stage >3, hypertension and antiphospholipid antibodies. There is also an increased risk of pre-eclampsia.
Extra-renal disease flare-ups are more common in the second and third trimesters, and postpartum flare-ups have been described.32
Flare-ups can be managed with corticosteroids. Maintenance
therapy with azathioprine and hydroxychloroquine is considered
safe. Tacrolimus and cyclosporin can be used in pregnancy with
therapeutic drug level monitoring. Differentiating between a
flare-up of lupus nephritis and pre-eclampsia may be
challenging. A low C3/C4, a rising double-stranded DNA (dsDNA)
and an active urine sediment with red cell casts are suggestive
of recurrence of lupus.
Drugs and the kidney
Methyldopa is the first-line agent of choice to treat hypertension in pregnancy and has a well-documented safety profile,33 , 34 while the calcium channel blocker, nifedipine,35 and the selective a1- and non-selective b-adrenergic receptor-blocker, labetalol, also appear to be safe.36 Diuretics are generally not recommended as they can theoretically compromise the pregnancy-associated increase in plasma volume, although outcome data have not supported these concerns.37 Angiotensin-converting enzyme (ACE) inhibitors are teratogenic and can cause renal dysgenesis, oligohydramnios, calvarial and pulmonary hypoplasia, intrauterine growth restriction, and neonatal renal failure, leading to death of the fetus as early as the first trimester.38 , 39 Similar anomalies have been reported with angiotensin II receptor blockers (ARBs).40 ACE inhibitors and ARBs should be discontinued prior to conception, with appropriate birth control measures while on these agents.
Calcineurin inhibitors,
glucocorticosteroids and azathioprine are the mainstay of safe
immunosuppressive therapy in pregnant transplant recipients
and lupus patients. Mycophenolate mofetil has been reported to
cause developmental toxicity, malformations, and intrauterine
death in animals, with growing evidence confirming its
teratogenic effects in humans. The drug should be discontinued
6 weeks before conception.
Conclusion
Because there are substantial physiological changes that occur
in a normal pregnancy, it is necessary to understand these in
order to determine whether the kidney developed disease during
pregnancy. Both pre-existing hypertension and renal disease
increase the risk of adverse pregnancy outcomes and increase the risk of
pre-eclampsia, which predisposes to preterm delivery.
Pregnancy in a patient with CKD may cause progression of renal
failure, and in the SA setting this is complicated by
restricted access to dialysis in the state sector. To ensure
the best outcome for mother and child, pre-pregnancy
counselling is essential. Attention must be paid to the
numerous teratogenic drugs that are commonly used to treat
hypertension and the immunosuppression necessary to treat
renal transplant recipients and those with glomerulonephritis
(e.g. lupus nephritis). Patients with renal disease and
hypertension are at high risk of complications and regular
antenatal assessments are required that should involve a
multidisciplinary team to monitor BP, proteinuria, diabetic
control and fetal wellbeing.
References
1. Rasmussen PE, Nielsen FR. Hydronephrosis during pregnancy: A literature survey. Eur J Obstet Gynaecol Reprod Biol 1988;27:249-259. [http://dx.doi.org/10.1016/0028-2243(88)90130-X]
2. Levey AS. Use of glomerular filtration rate measurements to assess the progression of renal disease. Semin Nephrol 1989;9:370-379.
3. Smith MC, Moran P, Ward MK, Davison JM. Assessment of glomerular filtration rate during pregnancy using the MDRD formula. Br J Obstet Gynaecol 2008;115:109-112. [http://dx.doi.org/10.1111/j.1471-0528.2007.01529.x]
4. Alper AB, Yi Y, Webber LS, et al. Estimation of glomerular filtration rate in preeclamptic patients. Am J Perinatol 2007;24:569-574. [http://dx.doi.org/10.1055/s-2007-986697]
5. Eknoyan G, Hostetter T, Bakris GL, et al. Proteinuria and other markers of chronic kidney disease: A position statement of the National Kidney Foundation (NKF) and the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Am J Kidney Dis 2003;2:617-622. [http://dx.doi.org/10.1016/S0272-6386(03)00826-6]
6. American College of Obstetricians and Gynecologists. Diagnosis and management of preeclampsia and eclampsia. Int J Gynaecol Obstet 2002;77:67-75.
7. Price CP, Newall RG, Boyd JC. Use of protein: Creatinine ratio measurements on random urine samples for prediction of significant proteinuria. A systematic review. Clin Chem 2005;51:1577-1586. [http://dx.doi.org/10.1373/clinchem.2005.049742]
8. National Committee on Confidential Enquiries into Maternal Deaths. Saving Mothers 2005-2007: Fourth Report into Maternal Deaths in South Africa. Expanded Executive Summary. Pretoria: Department of Health, 2009.
9. Magee LA, Ornstein MP, von Dadelszen P. Fortnightly review: Management of hypertension in pregnancy. Br Med J 1999;318:1332-1336. [http://dx.doi.org/10.1136/bmj.318.7194.1332]
10. Sibai BM. Treatment of hypertension in pregnant women. N Engl J Med 1996;335:257-265. [http://dx.doi.org/10.1056/NEJM199607253350407]
11. Chappell LC, Enye S, Seed P, et al. Adverse perinatal outcomes and risk factors for preeclampsia in women with chronic hypertension: A prospective study. Hypertension 2008;51:1002-1009. [http://dx.doi.org/10.1161/HYPERTENSIONAHA.107.107565]
12. Centre for Maternal and Child Enquiries. Saving mothers’ lives: Reviewing maternal deaths to make motherhood safer: 2006-2008. The Eighth Report on Confidential Enquiries into Maternal Deaths in the United Kingdom. Br J Obstet Gynaecol 2011;118(Suppl 1):1-203. [http://dx.doi.org/10.1111/j.1471-0528.2010.02847.x]
13. World Health Organization (WHO). World Health Report 2005: Make Every Mother and Child Count. Geneva: WHO, 2005:63.
14. Gu Y, Lewis DF, Wang Y. Placental productions and expressions of soluble endoglin, soluble fms-like tyrosine kinasereceptor-1 and placental growth factor in normal and preeclamptic pregnancies. J Clin Endocrinol Metab 2007;93:260-266. [http://dx.doi.org/10.1210/jc.2007-1550]
15. Venkatesha S, Toporsian M, Lam C, et al. Soluble endoglin contributes to the pathogenesis of preeclampsia. Nature Medicine 2006;12(6):642-646. [http://dx.doi.org/10.1038/nm1429]
16. Levine RJ, Lam C, Qian C, et al. Soluble endoglin and other circulating antiangiogenic factors in preeclampsia. N Engl J Med 2006;355:992-1005. [http://dx.doi.org/10.1056/NEJMoa055352]
17. Robinson CJ, Johnson DD. Soluble endoglin as a second-trimester marker for preeclampsia. Am J Obstet Gynecol 2007;197(2):174e1-5.
18. Rana S, Karumanchi SA, Levine RJ, et al. Sequential changes in antiangiogenic factors in early pregnancy and risk of developing preeclampsia. Hypertension 2007;50:137-142. [http://dx.doi.org/10.1161/HYPERTENSIONAHA.107.087700]
19. Walker JJ. Pre-eclampsia. Lancet 2000;356(9237):1260-1265. [http://dx.doi.org/10.1016/S0140-6736(00)02800-2]
20. Askie LM, Duley L, Henderson-Smart DJ, Stewart LA. Antiplatelet agents for prevention of pre-eclampsia: A meta-analysis of individual patient data. Lancet 2007;369:1791-1798. [http://dx.doi.org/10.1016/S0140-6736(07)60712-0]
21. Duley L, Henderson-Smart DJ, Knight M, King JF. Antiplatelet agents for preventing pre-eclampsia and its complications. Cochrane Database Syst Rev 2007;(2):CD004659.
22. Sibai BM, el-Nazer A, Gonzalez-Ruiz A. Severe preeclampsia-eclampsia in young primigravid women: Subsequent pregnancy outcome and remote prognosis. Am J Obstet Gynecol 1986;155:1011-1016. [http://dx.doi.org/10.1016/0002-9378(86)90336-4]
23. Vikse BE, Irgens L M, Leivestad T, et al. Preeclampsia and the risk of end-stage renal disease. N Engl J Med 2008;359(8):800-809. [http://dx.doi.org/10.1056/NEJMoa0706790]
24. Jones DC, Hayslett JP. Outcome of pregnancy in women with moderate or severe renal insufficiency. N Engl J Med 1996;335:226-232. [http://dx.doi.org/10.1056/NEJM199607253350402]
25. Hou S. Pregnancy in chronic renal insufficiency and end-stage renal disease. Am J Kidney Dis 1999;33:235-252. [http://dx.doi.org/10.1016/S0272-6386(99)70296-9]
26. Katz AI, Davison JM, Hayslett JP, et al. Pregnancy in women with kidney disease. Kidney Int 1980;18(2):192-206. [http://dx.doi.org/10.1038/ki.1980.128]
27. Leavey SF, Weitzel WF. Endocrine abnormalities in chronic renal failure. Endocrinol Metab Clin North Am 2002;31:107-111. [http://dx.doi.org/10.1016/S0889-8529(01)00006-8]
28. Krane NK. Acute renal failure in pregnancy. Arch Intern Med 1988;148:2347-2357.
29. Grünfeld J-P, Pertuiset N. Acute renal failure in pregnancy. Am J Kidney Dis 1987;9:359-362. [http://dx.doi.org/10.1016/S0272-6386(87)80137-3]
30. Rossing K, Jacobsen P, Hommel E, et al. Pregnancy and progression of diabetic nephropathy. Diabetologia 2002;45(1):36-41. [http://dx.doi.org/10.1007/s125-002-8242-4]
31. Mathiesen ER, Ringholm L, Feldt-Rasmussen B, et al. Obstetric nephrology: Pregnancy in women with diabetic nephropathy – the role of antihypertensive treatment. Clin J Am Soc Nephrol 2012;7(12):2081-2088. [http://dx.doi.org/10.2215/CJN.00920112]
32. Tandon A, Ibanez D, Gladman DD, Urowitz MB. The effect of pregnancy on lupus nephritis. Arthritis Rheum 2004;50:3941-3946. [http://dx.doi.org/10.1002/art.20638]
33. Montan S, Anandakumar C, Arulkumaran S, et al. Effects of methyldopa on uteroplacental and fetal hemodynamics in pregnancy-induced hypertension. Am J Obstet Gynecol 1993;168:152-156. [http://dx.doi.org/10.1016/S0002-9378(12)90905-9]
34. Sibai BM, Mabie WC, Shamsa F, et al. A comparison of no medication versus methyldopa or labetalol in chronic hypertension during pregnancy. Am J Obstet Gynecol 1990;162:960-966. [http://dx.doi.org/10.1016/0002-9378(90)91297-P]
35. Lindow SW, Davies N, Davey DA, Smith JA. The effect of sublingual nifedipine on uteroplacental blood flow in hypertensive pregnancy. Br J Obstet Gynaecol 1988;95:1276-1281. [http://dx.doi.org/10.1111/j.1471-0528.1988.tb06818.x]
36. Sibai BM, Mabie WC, Shamsa F, Villar MA, Anderson GD. A comparison of no medication versus methyldopa or labetalol in chronic hypertension during pregnancy. Am J Obstet Gynecol 1990;162(4):960-967. [http://dx.doi.org/10.1016/0002-9378(90)91297-P]
37. Collins R, Yusuf S, Peto R. Overview of randomised trials of diuretics in pregnancy. Br Med J (Clin Res Ed) 1985;290:17-23. [http://dx.doi.org/10.1136/bmj.290.6461.17]
38. Pryde PG, Sedman AB, Nugent CE, Barr M Jr. Angiotensin-converting enzyme inhibitor fetopathy. J Am Soc Nephrol 1993;3:1575-1582.
39. Cooper WO, Hernandez-Diaz S, Arbogast PG, et al. Major congenital malformations after first-trimester exposure to ACE inhibitors. N Engl J Med 2006;354:2443-2451. [http://dx.doi.org/10.1056/NEJMoa055202]
40. Saji H, Yamanaka M, Hagiwara A, Ijiri R. Losartan and fetal toxic effects. Lancet 2001;357(9253):363. [http://dx.doi.org/10.1016/S0140-6736(00)03648-5]
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