Here's how I think of it (not sure if right). Down syndrome: decreased AFP, decreased estriol, increased beta-HCG, and increased inhibin A.
1. AFP is made in the yolk sac/fetal liver and gets into the mother's blood where it is measured. Developmental delay in the down syndrome fetus = smaller yolk sac/fetus = less AFP getting into mother's blood.
2. Unconjugated estriol - kind of the same story. It is produced by fetus through its metabolism and crosses placenta to be measured in mother's blood. Developmental delay = less estriol.
Beta-HCG and Inhibin A seem much more complicated. Tried to look for mechanisms in some articles, but I'm a first year like you and haven't covered repro yet so I'm basically talking out of my ass. Interested in what other people can tell us because I was actually looking at this topic this morning.
