Trisomy Screening and hCG Levels

Biochemical Markers

The American College of Medical Genetics recommends that hCG is typically elevated in Down syndrome pregnancies, making it one of the key screening markers 1, 2
Free beta-hCG performs better than intact hCG for first-trimester screening, particularly at 11-12 weeks gestation, with a detection rate of 83-86% for Down syndrome at a 5% false-positive rate 1, 3
The elevation of hCG in Down syndrome is part of a characteristic pattern that also includes low AFP and low unconjugated estriol (uE3), which can detect 75-80% of Down syndrome cases with a 3% false-positive rate 2, 4
Elevated hCG combined with other markers achieves detection rates of 75-80% for Down syndrome in second-trimester screening, with a false-positive rate of 5% 2, 4

The National Society of Genetic Counselors notes that hCG is typically low in trisomy 18, which is the opposite pattern from Down syndrome, with a mean hCG of approximately 0.62 multiples of the median (MoM) 5, 6
Biochemical marker patterns in trisomy 13 are less well-characterized than for trisomy 21 or 18, but the clinical utility of biochemical screening for trisomy 13 is less certain compared to other trisomies 5, 6

The American College of Obstetricians and Gynecologists recommends that first-trimester combined screening (NT + PAPP-A + free β-hCG) achieves detection rates of 83-86% for Down syndrome at a 5% false-positive rate 3
Second-trimester quad screen detects approximately 75% of Down syndrome in women under 35 and over 80% in women 35 and older, with a false-positive rate of 5% 2, 4
Screening tests, including hCG, only provide risk assessment, not diagnosis, and only diagnostic testing (CVS or amniocentesis) provides definitive results, with a detection rate of 99% for Down syndrome 1, 4