Antenatal surveillance

Amniotic fluid is produced by fetal urination but, in the first 16 weeks of gestation, additional sources include the placenta, amniotic membranes, umbilical cord and fetal skin. Removal of amniotic fluid is by fetal swallowing.

Assessment of the amniotic fluid volume is usually made subjectively. Quantitatively, it is defined by the measurement of either the vertical distance of the largest single pocket of fluid free of fetal parts or the amniotic fluid index, which is the sum of the vertical measurements of the largest pockets in the four quadrants of the uterus

Oligohydramnios. The main causes are: rupture of the membranes, uteroplacental insufficiency, urinary tract malformations and the donor fetus in twin-to-twin transfusion syndrome

• The diagnosis of oligohydramnios is usually made subjectively by the absence of the ‘acoustic window’ normally provided by the amniotic fluid and the flexed posture of the fetus with lack of fetal movements. Quantitative criteria are: the largest single vertical pocket of less than 2 cm or the amniotic fluid index below the 5th centile of the normal range

Polyhydramnios. There are essentially two major causes of polyhydramnios:

• Reduced fetal swallowing, due to craniospinal defects (such as anencephaly), facial tumors, gastrointestinal obstruction, compressive lung and chest disorders, and fetal akinesia deformation sequence (due to neuromuscular impairment of fetal swallowing)
• (b) increased fetal urination, due to maternal diabetes mellitus, hyperdynamic fetal circulation secondary to fetal anemia, fetal and placental tumors or arteriovenous malformations and the recipient fetus in twin-to-twin transfusion syndrome

The vertical measurement of the largest single pocket of amniotic fluid is used to classify polyhydramnios into mild (8–11 cm), moderate (12–15 cm) and severe (16 cm or more)