This picture shows the physiological dilatation of the right ureter, in the middle third, compressed between the gravid uterus and the pelvic brim (Courtesy of Massimo Borri)
There are elevated baseline ureteral pressures in resting during pregnancy. Ureteral pressure has been recorded above the pelvic brim also during positional changes, and the resulting pressures were decreased. This reflect the displacement of the uterus from the ureter, which allows the reduction of the dilatation. Hydronephrosis of pregnancy usually does not occur in quadrupeds, whose uterus hangs away from the ureters .
Hormonal effects on the ureters have also been implicated, in order to explain the kidney dilatation during pregnancy. About 10–15 % of incidence of ureteral dilatation observed in the first-trimester, occurring before the uterus reaches the pelvic brim, supports a non-mechanical mechanism (Faundes 1989). This hypothesis may be endorsed by high levels of progesterone during pregnancy, which reduce ureter’s smooth muscle tone, peristalsis and contraction pressure. Progesterone has been proven to increase the degree of hydronephrosis during pregnancy, and to delay the rate of disappearance after birth , however it is not able by itself to cause ureteral dilatation. As a matter of fact, in women with pelvic kidney or ureteral diversion, in which the ureter enters the conduit above the pelvic brim, hydronephrosis is not observed. The significantly lower incidence of left-sided hydronephrosis also supports such hypothesis . Schneider et al. showed that administration of progesterone in non-pregnant women fails to cause hydronephrosis, hence there is no correlation between hormones and dilatation . Therefore, obstruction is the primary factor in the etiology of hydronephrosis during pregnancy and hormonal factors may be involved . Increased diuresis, small stones or other unrecognized factors may cause decompensation of ureteral function, progressing to symptomatic acute hydronephrosis . Nephrolithiasis is a relatively uncommon event in pregnancy, with frequency of 0.05 % .
Clinic and Imaging
Hydronephrosis of pregnancy is usually clinically silent. Renal colic is the most common non-gynecologic cause for abdominal pain and hospitalization during pregnancy and clinical presentation could vary from nausea, emesis, urgency, frequency, dysuria, hematuria and fever . Acute urinary stasis can lead to flank pain until pyelonephritis, renal failure, pre-eclampsia and pre-term labor. A normal curve of dilatation was proposed to define the upper limits of caliceal diameter during gestational weeks .
Ultrasonography is the imaging tool of choice for evaluation of hydronephrosis in pregnancy woman due to its relatively low cost, extensive results, real-time capability, safety [13, 14]. Typical ultrasonographic framework is hydroureteronephrosis extending to the pelvic brim. When ureteral dilation extends below it, a different etiology of obstruction, such as a ureteral stone, should be considered. In case the ultrasonography findings are questionable, Magnetic Resonance Imaging (MRI) will allow to make a distinction. On high-resolution T2 sequences, physiological dilatation will show the ureter in the middle third compressed between the gravid uterus and the pelvic brim (Fig. 2.2). Obstruction due to calculi causes renal enlargement and perinephric edema. When a stone is lodged in the lower ureter, a standing column of dilated ureter will be seen below the physiological constriction. The stone itself may be shown . In addition, MRI is useful to show also other causes of acute flank or abdominal pain, including appendicitis, ovarian torsion, and adrenal hemorrhage. The administration of gadolinium contrast should be avoided in pregnant patients because it is known to cross the placental barrier. Computed tomography (CT) is an acceptable alternative if there is a contraindication to MRI, but even low-dose regimes imply the use of ionizing radiation .
MRI sequence shows a right hydroureteronephrosis with a regular left kidney in a 30 weeks pregnancy woman. (a) T2 weight axial sequence (blue arrow shows right ureter dilatation,* shows fetal head). (b) T2 weight coronal sequence (Courtesy of Dr.ssa Federica Fiocchi, University of Modena and Reggio Emilia, Policlinico di Modena)
Management options are based on the coexisting stone disease, pyelonephritis and renal disease . The first line therapy in symptomatic women is conservative management (analgesia, bed rest, hydration and antibiotics when clinically indicated) unless the symptoms persist .