Normal Ranges for Double Marker Test in Pregnancy
The normal range for alpha-fetoprotein (AFP) in second trimester screening is below 2.0-2.5 multiples of the median (MoM) in singleton pregnancies, while normal human chorionic gonadotropin (hCG) levels are typically below 2.0-2.5 MoM. 1
Alpha-Fetoprotein (AFP) Normal Ranges
- AFP levels normally increase by approximately 10-15% per week during the second trimester 1
- For singleton pregnancies, normal AFP levels are below 2.0-2.5 MoM 1
- For twin pregnancies, normal AFP levels are below 4.0-5.0 MoM (approximately twice the singleton levels) 1, 2
- The optimal time for AFP screening is between 16-18 weeks gestation 1
- AFP levels can be affected by maternal weight, race, presence of insulin-dependent diabetes, number of fetuses, and family history of neural tube defects 1
Human Chorionic Gonadotropin (hCG) Normal Ranges
- Normal hCG levels are typically below 2.0-2.5 MoM in the second trimester 3, 4
- In Down syndrome cases, hCG levels are often elevated (higher than normal) 1
- In trisomy 18 cases, hCG levels are typically low 1
- Like AFP, hCG levels should be interpreted based on gestational age, with testing optimally performed between 16-18 weeks 1
Interpretation and Clinical Significance
- AFP values above 2.0-2.5 MoM may indicate neural tube defects, ventral wall defects, or other complications 1, 4
- Elevated hCG (>2.0-2.5 MoM) may be associated with increased risk of Down syndrome, preeclampsia, and preterm labor 1, 4
- Results must be interpreted in the context of gestational age, with reinterpretation needed if gestational age changes by 2 or more weeks after ultrasound dating 1
- Ultrasound dating of pregnancy improves accuracy of screening by reducing false-positive rates 1
Important Considerations
- Results are reported as multiples of the median (MoM), which adjusts for gestational age 1
- Each laboratory should establish its own normative data rather than relying solely on package inserts 1
- Screening performance is improved by expressing gestational age as weeks and days or decimal weeks rather than completed or rounded weeks 1
- Maternal weight adjustment is important as AFP levels are inversely related to maternal weight 1
- The double marker test is part of screening and not diagnostic; abnormal results require follow-up with genetic counseling, targeted ultrasound, and possibly amniocentesis 1
Common Pitfalls to Avoid
- Failing to adjust for gestational age when interpreting results 1
- Not considering maternal factors that can affect marker levels (weight, race, diabetes) 1
- Relying on screening alone when multiple markers are abnormal or structural anomalies are present 5
- Using the same cut-off values for twin pregnancies as for singleton pregnancies 1, 2
- Not reinterpreting results when gestational age is revised by ultrasound 1
Remember that while the double marker test provides valuable screening information, it has been largely replaced by triple or quad screening in many settings for improved detection rates of chromosomal abnormalities 1, 6.