Patient Profile
- Maternal age:
- 36 years
- Height / Weight:
- 168 cm / 65 kg (BMI 23.0)
- Gestational age:
- 12 weeks 3 days (CRL 60 mm)
- Pregnancy:
- G2P1 — singleton, spontaneous conception, no IVF
- Smoking / ethnicity:
- Non-smoker, Caucasian
- History:
- One prior uneventful term pregnancy. No family history of chromosomal abnormality. No diabetes.
Summary Overview
Combined first-trimester screening using ultrasound (NT) and maternal serum biochemistry (PAPP-A, free β-hCG). 4 inputs analyzed; biochemistry results are within typical ranges, NT is within normal limits, and the integrated risk falls below the standard screen-positive threshold (1:250).
Risk Calculation Inputs
- Maternal age-related background risk (T21):
- 1:212 at age 36 [Snijders RJ, Lancet 1999]
- NT MoM:
- 1.05 (within normal range)
- PAPP-A MoM:
- 1.12 (within normal range; low values associate with T21)
- free β-hCG MoM:
- 0.94 (within normal range; high values associate with T21)
- Combined detection rate at 1:250 threshold:
- ~85% for T21 with ~5% false-positive rate [Nicolaides KH, Am J Obstet Gynecol 2011]
Detailed Parameter Analysis
Risk Interpretation
-
Trisomy 21 (Down syndrome)Low — Screen Negative
Adjusted risk 1:1,420 — below the 1:250 standard cut-off. Background age-related risk (1:212) is reduced by reassuring biochemistry and NT.
-
Trisomy 18 (Edwards syndrome)Low — Screen Negative
Adjusted risk < 1:10,000. Both PAPP-A and free β-hCG are inappropriately low in T18 — neither is depressed here.
-
Trisomy 13 (Patau syndrome)Low — Screen Negative
Adjusted risk < 1:10,000. T13 typically presents with both biochemical depression and increased NT.
Analytical Summary & Recommendations
Clinical pattern
Reassuring combined first-trimester screen. All three components (NT, PAPP-A, free β-hCG) are within normal ranges and the integrated risk for the major aneuploidies is below standard screen-positive thresholds. Maternal age 36 raises the prior probability somewhat, but the screening result reduces the post-test probability significantly.
Important context
- Screen negative ≠ definitive normal. Combined first-trimester screening detects ~85% of T21; ~15% of cases will not be flagged by this approach alone [Nicolaides KH 2011]
- For higher detection (~99% for T21), cell-free DNA (cfDNA / NIPT) from maternal blood is an option starting from 10 weeks and is appropriate at any age [ACOG Practice Bulletin 226, 2020]
- Diagnostic testing (CVS or amniocentesis) provides definitive results but carries a small procedure-related miscarriage risk; reserved for screen-positive cases or when desired
- Anatomy ultrasound at 18–22 weeks remains the standard for structural assessment regardless of screening result
Recommended next steps
- Continue routine prenatal care
- Discuss whether NIPT would add value for additional reassurance (especially given maternal age and personal preferences)
- Schedule second-trimester anatomy ultrasound (18–22 weeks)
- Review obstetric history and screen for preeclampsia risk — separate clinical assessment recommended; PAPP-A is not depressed here, which is reassuring [Dugoff L 2004]
- Preserve serum sample temporarily in case integrated/sequential second-trimester testing is desired
Key Questions for Your Physician
- Given my age, would you recommend cell-free DNA testing in addition to this screen?
- What is the residual chance of a chromosomal condition not detected by this screen?
- How does this result interact with future screening (NIPT vs. quad screen vs. integrated)?
- Should we discuss preeclampsia risk modeling at this visit, given that PAPP-A also informs it?
- When is my next ultrasound, and what specific structures will it evaluate?
What This Means for You
Your first-trimester screening result is reassuring. The combination of your blood test (PAPP-A and free β-hCG) and the ultrasound measurement of the back of your baby's neck (NT) all fell within expected ranges, and the calculated chance of the three most common chromosomal conditions (Down syndrome, trisomy 18, trisomy 13) is low.
A "screen negative" result does not mean zero risk — this kind of test catches roughly 85% of Down syndrome cases. If you'd like additional reassurance, a cell-free DNA blood test (NIPT) is available and is more accurate for these specific conditions; it is a personal choice and worth discussing with your physician.
Your anatomy ultrasound at 18–22 weeks remains an important next step — it evaluates how your baby is forming, which is different information than what this screen provides.
A normal PAPP-A is also mildly reassuring for placental function later in the pregnancy.
High confidence in the risk calculation given complete inputs (correct CRL, all three biochemistry/sonographic markers, validated MoM conversion). Remaining uncertainty reflects the inherent ~15% false-negative rate of combined screening for trisomy 21.
References
- Nicolaides KH. Screening for fetal aneuploidies at 11 to 13 weeks. Am J Obstet Gynecol 2011;204(5):359–369.
- Snijders RJ, Noble P, Sebire N, Souka A, Nicolaides KH. UK multicentre project on assessment of risk of trisomy 21 by maternal age and fetal nuchal-translucency thickness. Lancet 1998;352:343–6.
- ACOG Committee Opinion / Practice Bulletin 226: Screening for Fetal Chromosomal Abnormalities. Obstet Gynecol 2020;136(4):e48–e69.
- Dugoff L, Hobbins JC, Malone FD, et al. First-trimester maternal serum PAPP-A and free-beta subunit human chorionic gonadotropin concentrations and nuchal translucency are associated with obstetric complications. Obstet Gynecol 2004;104(2):394–403.
- Souka AP, Von Kaisenberg CS, Hyett JA, et al. Increased nuchal translucency with normal karyotype. Am J Obstet Gynecol 2005;192(4):1005–21.
- Wright D, Spencer K, Kagan KK, et al. First-trimester combined screening for trisomy 21 at 7-14 weeks' gestation. Ultrasound Obstet Gynecol 2010;36(4):404–11.