Kidney Stones During Pregnancy: Imaging & Diagnosis

[Dr. Suzette Sutherland]
There's just so many worrisome details to consider, not only the urological details that we consider in the non-pregnant patient, but now we're worried about not only mom, but the fetus. Let's just start to dive into some basics about kidney stones and pregnancy. What's the real incidence of kidney stones during pregnancy? Is the incidence really higher? If so, what are the factors that increase that incidence?

[Dr. Alana Desai]
Sure. Previously thought to not be a higher risk in pregnant patients than the general population, because of the difficulties associated with managing them. The incidence may seem higher, but typically it's not higher than the general population. In a 2020 claims-based analysis of 1.4 million pregnancies, stone disease was found to be diagnosed in only 1% of all pregnancies.

There is some newer evidence from an observational study at Mayo Clinic in 2021, however, that pregnancy does increase the risk of a first-time symptomatic stone, with that risk peaking close to delivery. It does improve about a year after delivery, but there's still a bit higher risk compared to age-match controls going forward. It is also the most common non-obstructive because for hospital admissions, complicating 1 in 200 to 1 in 2,000 pregnancies.

[Dr. Suzette Sutherland]
That's a lot lower than I think most people think about. Certainly you don't see it all the time, but I guess maybe it just makes such an impression on us as urologists when we do see it in the pregnant patient that we think of it. Historically, people have thought that the incidence, maybe, or your chance of having a kidney stone as a woman is higher when you're pregnant because of maybe some hormonal changes or dilation of the ureters. Is any of that then true?

[Dr. Alana Desai]
There are both anatomical and physiological changes that occur during pregnancy due to the enlarging uterus. You can get just hydronephrosis, which typically occurs in about 90% of people by the third trimester, but it can also occur as early as a sixth week. That's just due to increased hormone production, namely progesterone. Because of this dilation, there's also an increased risk of stone migration and obstruction. Stones are twice as likely to be located in the ureter when diagnosed during pregnancy, just because of that migration.

Even when not caused by stones, hydronephrosis in pregnancy can also lead to symptoms. The dilation from the stasis of the urine along with the elevated progesterone causing further ureter dilation and reducing ureteral peristalsis, both of these can promote the formation of stone crystals or urinary crystals. This is further accelerated by the increased GFR, plasma, and the plasma flow rate. These do lead to an increased lithogenic factors, including urinary calcium oxalate and uric acid. The hypercalceria or higher calcium in the urine is also exacerbated by placental production of vitamin D to meet the demands of the fetus.

However, even though these lithogenic factors are increased by this increased flow, there is also an increased excretion of stone inhibitors such as glycoprotein, nephrocalcine, and tamhorsol that should compensate for the lithogenic factors. Again, a stone formation is not necessarily increased in pregnant patients because it is balanced by the stone inhibitors. Again, like you said, it potentially just feels like it's an increased risk in pregnant patients.

[Dr. Suzette Sutherland]
Is it maybe fair to say the incidence of developing a stone isn't necessarily higher in the non-pregnant female, but you described some other factors that might make them move during pregnancy are more likely that they try to start to pass. They may become more problematic during the pregnancy, whereas they were just nice and quiet up in the kidney previously. Is that fair?

[Dr. Alana Desai]
I think that is a potential. Again, with these findings from the Mayo Clinic, it's tough to know whether stone formation itself is increased or just a symptomatic event. Again, with that stone passage, like you said.

[Dr. Suzette Sutherland]
You used a term, I'm trying to think back to the term that you used, but hydronephrosis, I think of pregnancy is a term that we talk about. When you use that term, of course we could have hydronephrosis if we have an obstructing stone, but during pregnancy, are you just referring to the fact that the baby also is laying on the ureter during those later trimesters? Is there more to it in general, even during early pregnancy that it's common to see a slight hydronephrosis on one side or the other?

[Dr. Alana Desai]
Right. Both early in pregnancy, like I said, can occur up to as early as the sixth week of pregnancy. That's due to the increased progesterone that can be caused because of some of the ureteral dilation in itself. Then later in pregnancy is when hydronephrosis can be seen because of the gravid uterus itself. A mechanical obstruction, whereas in earlier pregnancy, it's this physiologic sort of response to the progesterone.

[Dr. Suzette Sutherland]
That's interesting because we often as urologists get referred these patients early on in their pregnancy. We know the fetus isn't that big yet. Pushing on the ureter causing the hydro, but we're being asked to evaluate them for a stent because they have some hydro. This is very important information to know that this is pretty common within six weeks already of pregnancy just because of hormonal changes.

[Dr. Alana Desai]
Exactly. Most people probably know this, but it is more common on the right side because of the production of the sigmoid colon on the left side. Up to 90% on the right side, it can occur in 67% on the left.

[Dr. Suzette Sutherland]
Then if we are suspecting that a pregnant woman, she's having some pain on one side or the other, how do we best differentiate between maybe they're just a little bit of hydronephrosis from the pregnancy versus a truly alarming, maybe obstructing stone? What's the best method of evaluation to start with?

[Dr. Alana Desai]
You have to start with your very basic history and physical examination. Again, as you said, you can't have leg pain just from pregnancy itself or back pain commonly. The most common presenting symptom of kidney stone disease is flank pain, about 80% to 100% present like this, followed by nausea and vomiting. Again, typical symptoms of pregnancy. You can also see other signs of stone disease such as microhematuria and gross hematuria, and then less commonly, fevers and chills.

Typically, again, you start with your basic history. Sometimes they'll tell you, I had an acute, sudden episode of pain in my back, and then it wrapped around to my abdomen, went to my groin. A little while later, I felt like I had to go to the bathroom. It just sounds like a classic stone history. That is very important. I think it's important to evaluate pregnant patients as you would a non-pregnant patient, taking those key events that happened during stone passage, and then, of course, considering the fact that there is a fetus involved. If there is fetal distress, then you have to take that into consideration. Initially, when you evaluate, it is much like a non-pregnant patient in taking that good history in laboratory evaluation as we normally would.