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The renal tract consists of two kidneys, which lie either side of the spine towards the back of the fetus. The kidneys each derive their blood supply directly from the aorta via the renal arteries, and drain blood in the inferior vena cava via the renal veins. The job of the kidney in the adult is to remove impurities by filtering and then concentrating the urine. In the fetus, this cleaning function is undertaken by the placenta, and the mother's kidneys, but in common with most of the other organs in the fetus, the kidneys start to function within the womb. The fluid produced is very dilute, and therefore is unlike urine, but is called fetal urine, and is the major source of the liquor within the womb.

The fetal kidneys start to produce "urine" from about 14 weeks gestation, and although this has no function in terms of filtering the fetal blood it is the major source of liquor (amniotic fluid) from 14 to 16 weeks onwards. Between 18 and 22 weeks gestation the fetus begins to "breath". Liquor can be seen using Doppler ultrasound moving in and out of the fetal lungs. This does not occur all the time, and has no function in terms of getting oxygen into the fetus. It is however vital to develop the complex channels and branches of the future airways within the lungs.


Renal tract dilation and Hyronephephrosis

Cystic dysplastic kidneys

Renal Agenesis

Other renal


© Perinatal Institute 2011