Pregnancy and urinary system

During pregnancy, the entire urinary system displays some anatomical (structural) and functional changes.

The anatomical changes are related to the fact that the size of the kidneys increases by 1-1.5 cm while their total volume increases by 30% and returns to the previous length 3-4 months after the childbirth. It is likely to rise due to the increase in the volume of the renovascular tumor, the diameter of its vessels and the intermediate space, without increasing its "histological" volume. Approximately 45-90% of the pregnancies show a "normal" pelvic stretch (the point within the kidney that collects the urine before it is pushed into the ureters) due to the progesterone and the pressure that the uterus exerts. There may even be a distension of the ureters (the tubules from the kidney to the bladder), particularly right. These changes are visible in ultrasound from the 2nd trimester and all this stretch can retain 200-300 cc. urine. As for the bladder, it becomes more relaxed and there is regurgitation of the urine from the bladder to the ureters. All the above comes back to a normal state 3 - 6 months after childbirth.

Functional changes include vasodilatation, blood pressure drop, increased cardiac output (the blood that the heart sends to the periphery), and increased renal perfusion due to pregnancy. This results in an increase in the GFR rate (e.g. increase in renal function) and hence the overall kidney function. GFR starts to increase in the 4th week of pregnancy and increases up to 50% in the first 2 months (GFR ~ 150 ml / min). The results of this increased function include the reduction of serum urea concentration (<20 mg / dl), serum creatinine reduction (<0.5 mg / dl), mild albuminuria [doubling the amount ~ 300 mg / 24h], reducing uric acid levels (from 6-12 ml / min to 12-20 ml / min) (3-4 mg /dl).

As for blood pressure, there is a reduction of about 10 mm Hg, rising to a peak in the 2nd trimester of pregnancy and a downward trend between week 18-24. Diastolic blood pressure decreases by 7-10 mmHg (in the 1st trimester) and returns to pre-pregnancy levels in the third trimester while systolic blood pressure is less varied.

The level of thirst is reduced in pregnancy resulting in fluid retention (6-8 L) and low sodium in the blood (hyponatremia). This is due to the increase in body water compared to sodium and as a result pregnant woman have normal Na + serum less than 5 mEq /L. Other laboratory changes are glucosuria (sugar in the urine) without the existence of diabetes mellitus.

  • Symptoms from the urinary tract during pregnancy

Among the most common complaints related to pregnancy are frequent urination and nycturia. They affect 80-95% of women at some point during pregnancy. We refer to frequent urination when the frequency of urination during the day increases (> 7 times) and in nighttime when it increases overnight (=> 2 times). Often these inconveniences start already in the first quarter. Nycturia increases as pregnancy progresses.

In addition, the incontinence and the urge to urinate may also occur due to uterine pressure in the bladder and to pregnancy-disturbed neuromuscular function. These disorders should be improved within 6 months of delivery.

These are "normal" adaptations of the urinary tract in general and the kidney especially in pregnancy. Their understanding is very important for a better interpretation of the pathological complications associated with renal dysfunction and pregnancy.

Mesogeios Dialysis Centers Group Scientific Team 

  • Bibliography 
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8. Comprehensive Clinical Nephrology, 5th Edition by Richard J. Johnson, MD, John Feehally, DM, FRCP and Jurgen Floege, MD, FERA

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