oligohydramnios

description

oligohydramnios is defined as a generalised decrease in amniotic fluid so that the vertical diameter of the largest pocket is less than 1 cm, or the largest pocket measured in two perpendicular planes is 1 cm or less, or the afi is below the 5th percentile for gestational age. oligohydramnios may also be defined as an afi less than 5.1 cm. normally amniotic fluid volume increases steadily from about 250 ml at 16 weeks of gestation to 800 ml at 28 weeks, 1000 ml at 34 weeks and declines to 800 ml at 40 weeks. the volume is determined by a steady state between input: fetal urination 400-1000 ml/da, alveolar exudate 600/900 ml/day, and outflow: fetal swallowing 250-450 ml/day and reabsorption through the chorioamniotic membranes 80 ml/day. input is affected by maternal blood flow to the placenta and the capacity for fetal maternal exchange at the intervillous space, modulated by maternal serum osmolality and maternal intervascular volume. oligohydramnios is associated with increased perinatal morbidity and mortality and develops in 0.5-4.0 percent of all pregnancies. it has been associated with congenital anomalies such as renal dysgenesis or obstruction, intrauterine growth retardation (iugr), post maturity and placental problems related to maternal fetal interaction. when oligohydramnios develops early in pregnancy, the fetal outcome is generally poor due to development of pulmonary hypoplasia, fetal compression syndrome, and possible amniotic band syndrome. barss et al. found second trimester oligohydramnios to be associated with no survivors in their series of twelve pregnancies. without adequate lung development, extrauterine life is impossible. premature rupture of membranes is a cause of oligohydramnios and occurs in approximately 10 percent of all pregnancies. it has been correlated with an increased incidence of perinatal death, fetal distress in labour, and infection. oligohydramnios associated with iugr may result from intrauterine hypoxemia which causes shunting of fetal blood flow away from the kidneys and lungs to more vital organs, the brain and heart. decreased urine production and alveolar transudate result in a decrease of amniotic fluid production. maternal hypertension may result in iugr from placental insufficiency. when hypertension is a manifestation of pre-eclampsia, reduction of the maternal intravascular volume may also be reflected by fetal volume contraction with resultant fetal oliguria and oligohydramnios. certain maternal drug therapies such as maternal indomethacin treatment, often used as an adjunct in treatment of premature labour may induce oligohydramnios. the mechanism is thought to relate to decreased renal perfusion.