Trisomy_18 site rankings:

anencephaly (4)
plexus (5)
trisomy 21 (6)
cyst (12)
infant loss (12)
adverse (13)
genetic disorders (76)
prenatal (77)
spina bifida (22)
diagnose (27)
negative (29)
sonogram (31)

trisomy 13 (2)
trisomy (6)
choroid (2)