Blood Test for Pregnancy Confirmation
The serum beta-human chorionic gonadotropin (β-hCG) test is the blood test used to confirm pregnancy in women. 1
Why β-hCG is the Gold Standard
β-hCG is produced by the implanting blastocyst and first appears in maternal blood approximately 6-8 days following fertilization, making it the most sensitive and specific marker for pregnancy detection. 2 This blood test can detect pregnancy earlier than urine tests and provides quantitative information useful for monitoring pregnancy progression.
Key Characteristics of β-hCG Testing
Serum β-hCG testing may be performed as needed for pregnancy confirmation or monitoring, and is recommended as part of comprehensive first trimester screening. 1
The test detects intact hCG, which is the major component in pregnancy serum, with levels rising rapidly to reach a peak at 7-10 weeks gestation. 2
Current generation tests are based on monoclonal antibodies to the beta-subunit of hCG, which virtually eliminate cross-reaction with pituitary luteinizing hormone (LH) and permit high sensitivity. 2
Clinical Applications Beyond Simple Confirmation
Quantitative Monitoring
Serial serum β-hCG measurements can predict pathologic pregnancies, with a normal rise of at least 66% in 48 hours or 120% in 72 hours indicating viable pregnancy. 3
β-hCG alone provides 78% sensitivity and 88% test efficiency for detecting pathologic pregnancies including ectopic pregnancy, spontaneous abortion, and blighted ova. 3
Additional Uses in Pregnancy Screening
In first trimester aneuploidy screening (11-14 weeks), hCG or free beta-hCG is measured along with pregnancy-associated plasma protein A (PAPP-A) and nuchal translucency, detecting approximately 70% of Down syndrome cases. 1
In second trimester screening, hCG is part of the quadruple marker test (with AFP, uE3, and inhibin-A) that detects approximately 75-80% of Down syndrome cases. 4, 5
Important Caveats and Pitfalls
Assay Specificity Issues
Different commercial assays measure different forms of hCG (intact hCG, total beta hCG, or free beta-subunit), and laboratories sometimes erroneously report intact hCG results as total beta hCG (9.3% error rate) or vice versa (13.1% error rate). 6
When hCG results do not fit the clinical picture, clinicians should measure hCG on a different assay, as some assays fail to detect all hCG isoforms or have problems with false-positive results. 4
Special Population Considerations
In women on maintenance hemodialysis, elevated β-hCG values can be found in non-pregnant women (32% of measurements), potentially indicating false-positive results that could erroneously suggest pregnancy. 7
Small amounts of hCG are present in blood of non-pregnant subjects, so clinical correlation is essential when interpreting borderline results. 2
Timing Considerations
For pregnancy confirmation, serum β-hCG can detect pregnancy as early as 6-8 days after fertilization, well before the missed menstrual period. 2
Accurate gestational age documentation is critical when using hCG for aneuploidy screening, as risk calculations require adjustment for maternal factors including weight, race, diabetes, and number of fetuses. 5