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Fetal pregnancy complications


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Hydronephrosis – which is swelling or dilation within the kidney or ureter – affects about 11 out of every 10,000 births and accounts for about 50% of birth defects.

This pathology can affect just one kidney, but when it affects both kidneys there is a poorer prognosis.

Antenatal hydronephrosis can be:

  • Physiological (no abnormality is noted after investigation in the postnatal period).
  • Caused by an obstruction of the passage of urine from the kidney into the ureter.
  • Caused by the reflux of urine from the bladder back to the kidney, which results in the dilation of the renal pelvis or calyxes and also sometimes the ureter. This back-flow of urine can be temporary and is benign in most cases.

In the majority of cases of hydronephrosis, there is a spontaneous improvement during the first two years of a child’s life.

If there is an obstruction of urine, it can be located in three places:

  • Where the renal pelvis meets the ureter (upper obstruction)
  • Where the ureter meets the bladder (upper obstruction)
  • At the outlet of the bladder into the urethra (lower obstruction)

Higher obstructions can resolve on their own, as in the case of reflux, however, lower obstructions will persist.

Bilateral hydronephrosis may be a sign of trisomy 21, so genetic counselling is needed to rule out the risk with amniocentesis.


The mechanism that causes hydronephrosis is difficult to determine, however, some of the causes of pathological hydronephrosis are the following:

  • Stenosis or obstruction of the ureteropelvic junction
    In 85% to 90% of cases, this complication affects a single kidney and affects male fetuses disproportionately at 65% of cases. This condition is almost always congenital and causes primary hydronephrosis, which is an obstruction in the ureter near the pyelon. Ultrasound can detect the possible occurrence of the condition but the diagnosis is confirmed via a nuclear medicine test after the birth of the child.
  • Vesicoureteral reflux
    This reflux is characterized by the back-flow of urine contained in the bladder to one or both kidneys. This complication is the result of a physiological abnormality in the anti-reflux system. In the majority of cases, this is caused by an insufficient angle of the ureter at the bladder, which prevents complete closure of the ureter and the anti-reflux system when the bladder contracts during urination. Reflux is classified into five grades, depending on the level of seriousness. The angle of the ureter into the bladder can eventually correct itself in the majority of grade 1 and 2 cases of reflux, which explains why the condition can resolve without intervention.
  • Posterior urethral valve obstruction
    Depending on the degree of obstruction, this can be a potentially serious complication. A membrane in the urethra slows down or even completely blocks the flow of urine, which affects the entire urinary system. When this occurs, dilation of the bladder can be observed with a thickening of the bladder wall and dilation of the kidneys and ureters (called megaureter).

In the most severe cases, there is increased pressure in the kidneys, causing reduced function. There also may be a decrease in amniotic fluid known as oligohydramnios. A complete lack of amniotic fluid, called anhydramnios, could cause more complications in the fetus, including poor lung development.

Symptoms to watch for

It is possible to see an obstruction of the posterior valve in the ultrasound toward the end of the first trimester along with signs of bladder dilation, called a megabladder. Prognosis is uncertain when such a diagnosis is made so early, especially if it is accompanied by oligohydramnios and kidney malformation.

During the second and third trimesters, signs of kidney complications that are visible on ultrasound are as follows:

  • Unilateral or bilateral hydronephrosis
  • An increase in volume and thickening of the wall of the bladder
  • A dilation of the ureters associated with a decrease in amniotic fluid (oligohydramnios) or an absence of fluid (anhydramnios)

Regular ultrasound follow-ups, depending on clinical course, is recommended to monitor renal dilation as well as the amount of amniotic fluid.

Tests and procedures

A referral to a university hospital centre is recommended in order to establish a diagnosis.

Ultrasound follow ups according to recommendations and case severity will determine where the baby should be delivered.

Treatments and follow up

The care of the mother and the unborn baby is carried out by a multidisciplinary team that includes an obstetrician specialized in maternal-fetal medicine, a geneticist, a nephrologist and/or urologist, a pediatric neonatologist, a radiologist, and a clinical nurse. Other professionals can also be part of the team, depending on each individual’s needs (a psychologist, for example).

Resources and useful links

Robert K. Creasy & Robert Resnik, Maternal-Fetal Medicine: Principle and Practice, 7th edition, Milton: Elsevier, 2014.


Page by

Dr Véronique Phan, Dr Lucie Morin et Chantal Larcher, inf. clinicienne

About this page
Updated on 1/25/2021
Created on 1/25/2021
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