Physiology of the Fetus Quiz

See Foetal Renal and Gastrointestinal Physiology. Option D is correct because the foetal kidneys begin producing urine from about 10 weeks, and by the third trimester foetal urine is the principal contributor to amniotic fluid volume. The foetus does not need its kidneys to clear waste, because the placenta does this, but a good urine output is essential to maintain the volume of amniotic fluid. In turn the fluid is needed for normal lung development and also protects the umbilical cord from being compressed. The fluid is constantly recycled: the foetus swallows it, which is important for normal gut development, and it also passes through the lungs and is absorbed by the gut, while urine adds new fluid. Conditions that lower urine output therefore lead to oligohydramnios, a low volume of amniotic fluid, and its complications; bilateral renal agenesis (absence of both kidneys) and posterior urethral valves (a blockage to urine flow out of the bladder) are examples. In the opposite direction, conditions that impair swallowing, such as oesophageal atresia, cause polyhydramnios, too much fluid. The foetus drinks the fluid and passes it back as urine, so the kidneys keep the supply topped up.

• Source of fluid: by the third trimester foetal urine is the principal contributor to amniotic fluid volume.
• Start of urine: the foetal kidneys begin producing urine from about 10 weeks.
• Low output: conditions that lower urine output cause oligohydramnios, a low volume of amniotic fluid.