Table 3.

Markers used for prenatal screening in trisomy 21.

Hormone	Typical Expression	First Trimester in T21	Second Trimester in T21
βhCG	- Detectable at 4 weeks	↑↑	↑↑
	- Peaks at 10 weeks
	- Declines until 20 weeks
	- Constant until term
PAPP-A	- Detectable at 8 weeks	↓↓	↔
	- Steady increase until term
AFP	- Detectable at 12 weeks	↓	↓
	- Peaks at 32 weeks
	- Declines until term
uE3	- Detectable pre-implantation	↓	↓
	- Steady increase until term
DIA	- Detectable at 5 weeks	↑	↑↑
	- Peaks at 8 weeks
	- Steady decrease from 8 to 16 weeks
	- Steady increase until 36 weeks
Activin-A	- Detectable at 8 weeks	↔	↔
	- Constant level throughout 1st and 2nd trimester
	- Steady increase from 24 weeks to term
PlGF	- Increases steadily throughout gestation	↓	↓
	- Peaks at 30 weeks
	- Steady decrease until term
Cell-free DNA	- Increases throughout gestation	↑	↑
	– 0.1% increase/week from 10 to 20 weeks
	– 1% increase/week from 21 weeks to term

↓ or ↑ = 25% difference, ↓↓ or ↑↑ = 50% difference, ↔ distribution overlaps with normal population.