During pregnancy, there are a number of normal physiologic changes that occur in the kidney:
- The kidneys increase 30% in size, from increased vascularity and interstitial space.
- The GFR increase 40%, which is reflected in a normal drop of serum creatinine to 0.4-0.8 during pregnancy. In other words, a creatinine of 1.0, normal in a non-pregnant woman, is considered abnormally high during pregnancy. Similarly, BUN normally drops to 8-10 during pregnancy.
- The renal pelvis and ureters become soft and dilated, able to hold several hundred cc of urine.
- Some mild glucosuria during is common since the glucose load presented to the kidneys frequently exceeds the renal threshold for reabsorbtion.
- The urine has a somewhat higher pH, with increased levels of bicarbonate.
The incidence of asymptomatic bacteruria is usually the same, pregnant or not pregnant, but the consequences during pregnancy are more significant. Because of the physiologic changes of stasis, alkaline urine, and glucosuria both lower and upper urinary tract infection risk is increased. We screen all pregnant women are screened for asymptomatic bacteruria at the initial prenatal visit, and for those at high risk, we may screen again after mid-pregnancy. Treatment of lower and upper urinary tract infections during pregnancy is the same as when not pregnant.
Nephrolithiasis or stone formation risk is about the same during pregnancy as when not pregnant. The diagnosis is based on the same criteria as when non-pregnant. To avoid x-rays, ultrasound can be used to identify hydroureter and sometimes the stone, but is only about 60% sensitive, and its specificity is limited by the normal, physiologic ureteral changes. When needed, x-rays may be safely used on a limited basis to identify the stone in questionable cases. The standard therapies of hydration, analgesia, urine straining and antibiotics for those with infection are also used during pregnancy.
Up to 85% of symptomatic stones will pass spontaneously during pregnancy, in part due to the normally dilated ureters. For those that do not, a number of options are available:
- Ureteroscopic stone removal
- Ureteroscopic laser lithotripsy
- Ureteral stent placement
- Shock wave lithotripsy is contraindicated during pregnancy, although for the few reported cases of inadvertent use during pregnancy, the pregnancy outcomes have been good.
For women with pre-existing renal disease, the disease will generally remain about the same or get worse. If it worsens, the primary risks to the mother will be renal failure and hypertension. For the pregnancy, the primary risks are pre-eclampsia, prematurity, and fetal growth abnormalities.
The prognosis for renal deterioration during pregnancy is reflected by the pre-pregnancy serum creatinine:
- Creatinine < 1.5, prognosis good
- Creatinine 1.5-3.0, prognosis guarded
- Creatinine > 3.0, may have serious deterioration of renal function
For women on dialysis, prematurity is common, even after optimizing all of the controllable factors. In one large study, the mean gestational age at delivery was 30.5 weeks. Prematurity, in combination with the problem of pre-eclampsia has led many centers to report neonatal survival rates of about 50% among infants born to women on dialysis.
The outlook is better for women with a transplanted kidney, in whom a pregnancy success rate of better than 90% is expected. The pregnancy will not have an adverse effect on the transplanted kidney, although depending on where it is located, it may or may not obstruct labor, requiring a cesarean section. Some immunosuppressive drugs are acceptable for use during pregnancy and others are not. It is best to manage these patients in a setting where expert consultation is readily available.