Kidneys in pregnancy
The impact of pregancy on renal disease.
THERE are many changes in the body during pregnancy, and the kidneys are no exception.
The blood flow to the kidneys is increased by 50-70% in pregnancy, with the most obvious changes in the first two trimesters, i.e. up to 28 weeks of pregnancy.
This results in an increase in the glomerular filtration rate with resulting decrease in blood levels of blood urea and creatinine, which are markers of glomerular function.
So normal levels of these two compounds during this period may indicate renal disease.
Blood pressure decreases by about 10 mm Hg in the initial 24 weeks of pregnancy, and then gradually returns to pre-pregnancy levels by 40 weeks.
As such, a blood pressure greater than 120/80 mm Hg may be a sign of the pregnant woman’s predisposition to high blood pressure (hypertension).
There is also a change in the function of the renal tubules, with increased loss of sugar into the urine (glycosuria), making the screening of diabetes more problematic.
The kidneys increase in size by one to 1.5 cm in pregnancy, and may stay that size for up to 12 weeks after delivery.
The pelvis of the kidneys and the ureters increase in size (dilate), presumably under the effects of progesterone produced by the placenta.
This dilatation is greater on the right, due to the rotation of the uterus to the right and the dilatation of the ovarian veins.
These changes may lead to collections of urine (stasis), which increases the likelihood of urinary tract infections.
In general, the functional changes in the urinary tract in pregnancy reaches its maximum effects by the end of the second trimester, and then return to the pre-pregnancy levels.
The anatomical changes, however, return to pre-pregnancy levels only up to three months after delivery.
Urinary tract infection
The dilated collecting system of the urinary tract, urinary stasis and backflow of urine from the bladder into the ureters (vesicoureteral reflux) in pregnancy increases the likelihood of bacteria colonising the urine (bacteriuria).
Pregnant women with bacteriuria, but with no symptoms, have an increased risk of urinary tract infection (UTI), which can lead to an infection of the kidneys (pyelonephritis) in about onethird of those affected.
Antibiotic treatment of UTI has to be selective as some antibiotics cannot be prescribed in pregnancy, e.g. trimethoprim-sulfamethoxazole.
An acute infection, or pyelonephritis, may have to be treated in hospital with intravenous antibiotics and fluids.
Some protein may be present in the urine in a normal pregnancy.
However, amounts larger than 300mg per day are signs of preexisting kidney disease that has become worse, kidney disease that occurred in pregnancy, or the development of pregnancy hypertension, especially if it occurs after 20 weeks of pregnancy.
Acute renal failure
Although rare, acute renal failure does occur in pregnancy.
The causes may be similar to those in non-pregnant patients, but there are specific disorders of pregnancy which can lead to acute renal failure.
They include excessive and inadequately treated vomiting (hyperemesis gravidarum), shock caused by bleeding from spontaneous and/or septic abortion, acute pyelonephritis, severe pre-eclampsia, eclampsia, HELLP (haemolysis, elevated liver enzyme levels, low platelet count) syndrome, uterine bleeding with separation of the placenta from the uterus (abruptio placentae), severe post-partum haemorrhage, disseminated intravascular coagulation and acute fatty liver of pregnancy.
Other causes include urinary tract stones and antiphospholipid syndrome.
The management of acute renal failure includes treating the underlying cause and dialysis.
High blood pressure (hypertension) is a common complication of pregnancy.
Pregnancy hypertension is a significant cause of maternal and foetal morbidity and mortality.
Pre-eclampsia (PET) refers to hypertension and proteinuria in women pregnant for the first time (primigravida), whose blood pressure was previously normal. It usually occurs after 32 weeks of pregnancy, although it may occur earlier.
PET resolves within 10 days after delivery of the baby. It occurs in primigravida and in older women who have had previous pregnancies. PET is more likely to occur in those with pre-existing hypertension, diabetes and in a twin pregnancy.
Eclampsia refers to the occurrence of seizures in those with pre-eclampsia. Chronic hypertension occurs in women who have a prepregnancy blood pressure of 140/90 mm Hg or more prior to 20 weeks of pregnancy. The likelihood of pre-eclampsia, abruptio placentae, intrauterine growth retardation and foetal death is increased in those with chronic hypertension.
Pre-eclampsia can also be superimposed on chronic hypertension. It occurs in women who have hypertension prior to 20 weeks of pregnancy and develop a sudden increase in blood pressure, proteinuria and other biochemical abnormalities. It occurs more often in older women or those who have some renal disease.
As PET is resolved with the delivery of the baby, a balance has to be struck between allowing the foetus to mature as much as possible inside the mother, and not compromising on the mother’s safety.
Eclampsia and HELLP syndrome are indications for immediate delivery, irrespective of the duration of pregnancy.
Certain medicines used in the treatment of hypertension cannot be prescribed in pregnancy.
Pre-existing kidney disease
Women with kidney disease can, and do, get pregnant.
The impact on patients with chronic renal disease has to take into account the effects of pregnancy on the kidneys as stated earlier, and the effects of kidney disease on pregnancy.
There is a deterioration of renal function in some patients with chronic renal disease. The degree of dysfunction at the time of conception, and the presence and extent of hypertension and proteinuria, influence the patient’s outcome significantly.
Generally, fertility is decreased considerably in women with chronic renal disease.
However, sometimes pregnancy occurs even in women on dialysis, but the risks of intrauterine growth retardation, pre-term labour and foetal loss are increased, although foetal survival rates are about 95% in most studies.
The progression of the underlying maternal kidney disease is dependent on its severity, rather than the specific disease.
Pregnant women with diabetic nephropathy (kidney disease) may develop worsening hypertension and proteinuria.
Those with systemic lupus erythematosus (SLE) may develop worsening hypertension and proteinuria, which leads to problems in differentiating the cause of the problem from PET.
The medicines that are usually used in treating SLE cannot be prescribed in early pregnancy because of the risk of causing foetal abnormalities.
Transplantation leads to a return of fertility. Although most women with kidney transplants deliver successfully, there is an increased risk of spontaneous abortion, ectopic pregnancy, preterm delivery, low birthweight babies, stillbirth and neonatal (babies aged one month and below) death.
Other challenges include use of immunosuppressive medicines (drugs that lower the immune system), which increase the risk of hypertension and opportunistic infections (infections that occur because the patient has a weak immune system), some of which – e.g. cytomegalovirus and toxoplasma – poses risks to the foetus.
Women who develop kidney failure in pregnancy face more challenges than non-pregnant patients.
Longer and more frequent dialysis is associated with better foetal outcomes.
The management of pregnancies in patients with pre-existing renal disease requires a team approach involving obstetricians, physicians, paediatricians, nurses and other health care professionals.
Pre-pregnancy counselling and skilled obstetric management have a major impact on the success of a pregnancy or otherwise.
Counselling helps in the selection of an appropriate time for pregnancy; for example, when there is stability in the renal function and good control of the blood pressure.
It would be helpful if there is discontinuation or reduction in dosages of certain medicines, e.g. steroids and immunosuppressives.
Measures to reduce the likelihood of infections like rubella, hepatitis B, toxoplasma and cytomegalovirus, among others, would also be helpful.
The major cause of morbidity and mortality in pregnant patients with renal disease is preterm delivery.
As with PET. the challenge for obstetricians is to strike the balance between allowing the foetus to mature as much as possible, without compromising on the mother’s safety. n Dr Milton Lum is a member of the board of Medical Defence Malaysia. This article is not intended to replace, dictate or define evaluation by a qualified doctor. The views expressed do not represent that of any organisation the writer is associated with.