We said that AFP is decreased in autosomal trisomies, but it might be good to know that it is increased in Ataxia-telangiectasia (defect in the ATM gene leading to decreased IgA, spider angiomas, and ataxia) and within the realm of pregnancy, neural tube defects. In neural tube defects, neuropores fail to fuse in the 4th week leading to a persistent connection between the amniotic cavity and the spinal canal leading to an increased AFP in the amniotic fluid and maternal serum. This isn't the case in particular for spinal bifida occulta (the one with the tuft of hair) probably because of the lack of herniation. Remember that these diseases can also cause polyhydramnios (mainly anencephaly) due to the lack of a swallowing center in the brain.
AFP is also a tumor marker for Yolk sac tumors, mixed embryonal carcinomas (low yield), and hepatocellular carcinoma.
submitted by โshak360(23)
This page from FA2020 and other resources show that certain lab values in pregnancy screening are diagnostic for Trisomy 21.
The easy thing to remember is that in all autosomal trisomies PAPP-A is decreased. (In trisomy 21 Down, 18 Edwards, and 13 Patau, PAPP or pregnancy-associated plasma protein is decreased).
For Down syndrome specifically, first-trimester ultrasound commonly shows increased nuchal translucency and hypoplastic nasal bone.
Finally, in trisomy 21 Down syndrome, the markers are HI up, as in
In other trisomies, these values are decreased.