1st trimester - most from amniotic mem, electrolytes transported in then water then follows freely
    Chorion Frondosum - site of free exchange with blood & amnion
    Fetal skin - freely exchanges fluid until keritinization at 24wks
    Alveoli & GI tract continue exchange throughout gestation, with lungs max production ~24wks of 340mL/day with swallowing of 170 mL/day
    like urine, it becomes increasingly hypotonic to serum with decreased K+ & Na+ w increased urea & creatinine

    25wks - fetal urine output 100ml/d, up to 600ml/d at term
        Normal volume 60ml at 12wks, 200ml at 16wks, 900ml at term
            post term vol dec to 500ml

Amniotic Fluid Index - 4 pockets measured in cm, one in each quadrant (ROT: Normal range 10-20cm throughout pregnancy)
        vertical height of all 4 totaled, any total <5 is below normal

Polyhydramnios - 60% idiopathic
    Maternal Factors - diabetes & erythroblastosis fetalis seen in 75% of hydropic fetuses
    Placental Factors - Chorioangioma (hemangioma)
    Fetal Anomaly - prox GI obs, neural tube defect, hydrops
        #1 fetal cause of severe polyhydramnios is CNS abnormality

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