correct answer
d fetal urine flow
Anhydramnios is a lack of amniotic fluid surrounding the developing fetus defined as the deepest pocket of amniotic fluid measuring two cm or less in the second trimester. Anhydramnios due to fetal renal or urinary tract anomalies may result in severe pulmonary hypoplasia. Amnioinfusion, an intervention designed to restore amniotic fluid volume in pregnant women who carry a fetus with such anomalies, may be a way to prevent pulmonary hypoplasia.
The biology of amniotic fluid production and reabsorption is not well understood. In normal pregnancies, the amount of amniotic fluid increases throughout pregnancy and peaks around 33 weeks gestation. Normal amniotic fluid volume depends on the fetus’ ability to swallow appropriately, produce normal lung secretions, and urinate. Any disruption in these functions will lead to abnormal amniotic fluid volume (e.g. polyhydramnios in esophageal atresia or oligo- or anhydramnios in lower urinary tract obstruction). The amniotic fluid volume is completely replaced every 24 hours.
The fetus begins to form urine around eight weeks of gestation with critical lung development occurring between 16 and 20 weeks of gestation. Most cases of anhydramnios are not found until the pregnant woman undergoes a full fetal anatomic survey ultrasound between 18 and 20 weeks. By this time, irreversible damage has occurred to the developing kidney. The risk for long term kidney dysfunction is high and is worse for children with hydronephrosis associated with lower urinary tract abnormalities and dysplastic kidneys compared to those with isolated hydronephrosis. Bilateral renal agenesis occurs in 1/3,000 to 5,000 births.
Antenatal ultrasound is the best initial choice to further evaluate renal and urogenital anomalies when anhydramnios is identified. The presence and location of the kidneys as well as abnormal echogenicity, cysts or urinary tract dilation should be documented.
In 2017, Polzen et al reported outcomes of eight women who underwent placement of a total of nine amnioports. They state that amniotic volume was restored in all patients. Five of the eight children died postnatally, one from an unrecognized laryngeal web, one from severe pulmonary hypoplasia, and three from renal failure due to renal parenchymal malformation on day of life one, nine and 94. All three children that survived had lower urinary tract obstruction from posterior urethral valves and received a kidney transplant at 19, 22, and 24 months of age.
A prospective, non-randomized trial of amnioinfusion for fetuses with bilateral renal agenesis demonstrated that serial amnioinfusions prevented lethal pulmonary hypoplasia but only 6 of the 17 live births survived to hospital discharge.
