·         Pelvic kidney - When one renal fossa is empty, a search for the kidneys in ectopic positions should be made. If the contralateral kidney is of normal size the obvious place to search for a kidney is the pelvis. Ectopic kidneys may be associated with other urinary tract anomalies.

 

·         Renal agenesis- Bilateral renal agenesis is relatively common: 1:3000—4000.

Severe oligohydramnios is seen.

 

·         Hyoiplastic kidney- Renal hypoplasia may be unilateral or bilateral and the small kidney may be difficult to locate. The contralateral kidney may be hypertrophied but if it is also small then there may be OH. Renal hypoplasia may also be associated with other anomalies.

 

·         Crossed renal ectopia- A kidney with a bibbed contour, with two areas of increased echogenicity representing the collecting systems, is seen in one renal fossa. A kidney in the contralateral renal fossa would not be seen. As expected the crossed ectopic kidney will be longer than a normal kidney.

 

·         Horse shoe kidney- May be associated with Turner’s syndrome. The kidney is more inferiorly placed and therefore the renal fossa may appear empty.

 

NB- A prominent adrenal gland or bowel loop in the renal fossa may mimic the kidney, making diagnosis difficult. The normal fetal kidney after 22 weeks gestation will show corticomedullary differentiation.