aneuploidy: abnormal number of chromosomes, increases with maternal age
triploidy: extra set of chromosomes (69)
trisomy: extra pair of chromosomes (47)

Down's risk 1/910 at 30yrs, 1/110 at 40, 1/240 at 37.
          amnio risk ~ 1/200

Alpha fetoprotein
    glycoprotein made initially by yolk sac, then liver.
    peaks at 28-32 wks
    fetal tissue not covered by skin leaks AFPinto blood that can exceed median by 2.5X [multiple of median, MOM] (singleton), or >4.5MOM for multiples
    elevated >98% in open spina bifida and anencephaly. Abdominal wall defects (gastroschisis [90%], omphalocele [40%], Placental anomaly: chorangioma, hematoma, insufficiency; Chromosome anomalies: Trisomy 13, 18, Turner's, Triploidy
    low levels (0.63< MOM<1) is asssociated with increased Down's risk
       sensitivity for Downs is only 21%

Triple screen detects 50% of Down's
    alpha fetoprotein plus:
    human chorionic gonadotropin: produced by placental synctiotrophoblasts and is higher in Down's suggesting placental hypersecretory immature placenta
    unconjugated estriol: DHEAS from fetal adrenal gland converted in fetal liver and unconjucated in placenta by sulfatase is found to be 25-30% lower in Down's
      

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