Along with the other challenges faced by women while they are pregnant, you can add the risk of developing a kidney stone.
The risk of stone formation during pregnancy.
A woman’s physiology changes dramatically during pregnancy, which can influence her chances of developing a new stone. While some changes increase a woman’s likelihood of forming a new stone, others decrease it. The end result is that the rate of stone formation during pregnancy actually appears similar to the rate in non-pregnant women and has been estimated to occur in 1 in 1500 pregnancies.
Changes during pregnancy that have an effect on stone formation:
- A woman’s cardiovascular system increases it output.
- Her kidneys increases their filtration activity.
- More calcium is absorbed by the intestines and more is released into the urine.
- Other urinary substances also increase including citrate, which helps prevent stones.
- The upper urinary tract (including kidneys and ureters) become dilated due to compression from the uterus and the effects of hormones. This is more pronounced on the right side and can lead to slower transport of urine and higher chances of infection or stone formation.
When do stones present during pregnancy and what are the chances of spontaneous passage?
Researchers in France have recently compared 244 pregnant women who formed stones with 5,712 non-pregnant female stone formers (Meria and coworkers). They found that stone episodes during pregnancy occurred primarily during the second and third trimesters, which accounted for 39% and 46% of the episodes. Encouragingly, 81% of the pregnant women were able to successfully spontaneously pass their stones in their group of patients, a much higher success rate than in the non-pregnant women (47%). Stones in the pregnant women were more likely to be calcium phosphate in nature rather than the more common calcium oxalate stones seen in the general population, reinforcing the fact that stone formation during pregnancy is a result of unique pregnancy related changes in physiology. More recent research published by Burgess and colleagues from Minnesota also found a high percentage of calcium phosphate stones but reported a much lower successful spontaneous passage rate of only 48%.
Diagnosis and treatment of stones during pregnancy.
The diagnosis and treatment of kidney stones during pregnancy can be more difficult because the need to limit radiation from x-rays to the growing baby makes diagnosing stones less certain and concerns about potential health risks of stone surgery when a woman is pregnant can restrict the surgical options available.
Radiation exposure from x-rays or CT scans used to diagnose a stone is more of a risk to the fetus during the first trimester. While the risk is lower during the second and third trimester, experts do not agree on whether there is a “safe” level or radiation and the philosophy of avoiding unnecessary radiation and minimizing it when it is unavoidable is advisable. This strategy includes using ultrasound or MRI initially to establish a diagnosis. However, these studies are not as accurate as CT for the diagnosis of stones. Because an undiagnosed stone can carry its own risks to the mother and fetus (pain, infection, preterm labor, and hypertension), in certain cases, the risk of obtaining imaging in order to treat a stone may be justified. Low dose CT scan protocols or plain x-rays may be used in these situations to still limit the amount of radiation exposure.
Once a stone is diagnosed, the decision to treat or observe it depends on factors such as whether pain is uncontrolled, infection is present, or kidney function is impaired. When these conditions are not present, a trial of passage or observation with planned treatment after delivery should be considered first because half or more of these stones will pass spontaneously. If intervention is needed, the options include placement of a ureteral stent or nephrostomy tube. If a stent is chosen, it may need to be changed every 2 months or more frequently because of the faster development of stent encrustation (stone particles forming on a stent) that occurs in pregnant women. A nephrostomy tube, placed through the skin directly into the kidney, avoids this issue but is associated with the inconvenience of requiring an external drainage bag. Treatment during pregnancy is usually limited to ureteroscopy with laser lithotripsy. Shockwave lithotripsy is not performed because of risks from the shockwaves on the developing fetus and percutaneous nephrolithotripsy is avoided because of the belly down position necessary for surgery. Both procedures also require a moderate amount of undesirable x-rays.
Steps you can take.
The most important step you can take to help prevent the development of a kidney stone while you are pregnant is to keep your water intake high. This will make your urine more dilute and make it less likely a new stone will form.
It is important to seek professional evaluation and care if you think you may be suffering from a stone. Typical symptoms of a stone during pregnancy include nausea, vomiting, blood in the urine, and flank or abdominal pain.
Thomas et al, “Urologic emergencies during pregnancy.” Urology, 2010.
Meria et al, “Stone formation and pregnancy: pathophysiological insights gained from morphoconstitutional stone analysis.” Journal of Urology, 2010.
Burgess et al. “Diagnosis of Urolithiasis and Rate of Spontaneous Passage During Pregnancy.” Journal of Urology, 2011.