Anatomical and Physiological Modifications During Pregnancy

During pregnancy, an increase in vascular volume, renal output (+60%), and glomerular filtration rate (+40%) is noted. Other than a 1-cm increase in the size of the kidneys, these changes result in an increase in the rate of filtered creatinine, urea, sodium, calcium, and uric acid (Biyani and Joyce 2002a). Hypercal-ciuria is induced by the decrease in the production of parathormone and by an increase in the 1-25 OH-D3 produced by the placenta, which is responsible for an increase in the intestinal absorption of calcium. Despite hypercalciuria and physiological hyperuricu-ria, the incidence of calculi does not rise during pregnancy, since the rate of factors inhibitory crystallization (citrate, magnesium, glycoproteins) is also higher (Biyani and Joyce 2002a; Meria et al. 1995). Urine, more alkaline because of respiratory alkalosis, opposes the formation of uric acid stones despite hype-ruricuria.

Physiological dilatation of the upper urinary tract is found in more than 90 % of pregnant women. This dilatation occurs between the 6th and 10th weeks and disappears 4-6 weeks after delivery (McAleer and Loughlin 2004). For anatomical reasons, it predominates on the right side. Different theories seek to explain this dilatation:

• The hormonal theory involves the inhibiting role of progesterone on the ureteral smooth musculature (Biyani and Joyce 2002a; Saidi et al. 2005). This theory is supported by experimental studies that have shown that administering progesterone to the female rat increases ureteral dilatation. This has not been confirmed by other authors. The hormonal theory does not explain the predominance of ure-teral dilatation on the right side. It undoubtedly plays an accessory role in the first months of pregnancy (Biyani and Joyce 2002a; McAleer and Loug-hlin 2004).

The mechanical theory involves the compressive role of the uterus, with this effect predominating on the right because of the uterus's dextrorotation. Ureteral compression by the ovarian vein and by the dilated uterine veins has also been suggested. The protection of the left ureter by the sigmoid reinforces the asymmetric character of the dilatation (Chaliha and Stanton 2002; Gorton and Whitfieldd 1997; Grenier et al. 2000). The absence of ureteral dilatation in cases of pelvic kidney, after ileal conduit urinary derivation, or in the quadruped confirms the involvement of mechanical phenomena in this dilatation (Biyani and Joyce 2002a).

Physiological dilatation during pregnancy is sometimes the cause of painful symptoms that usually regress with the use of mild analgesics. The persistence of pain or the appearance of infectious signs require urine drainage by a ureteral drainage stent or a percutaneous nephrostomy (Puskar et al. 2001).

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