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Difficulty: HardNormal Pregnancy and Prenatal Care

An obstetrician is evaluating a 2424-year-old woman during her first prenatal visit. She is gravida 11, para 00. She reports having irregular menstrual cycles and states that her last menstrual period began exactly 1616 weeks ago. She has no significant past medical history and takes no medications other than a daily prenatal vitamin. Vital signs are within normal limits. A pelvic ultrasound is performed in the office and demonstrates a single viable fetus with biometric measurements consistent with a gestational age of 1414 weeks and 33 days. The patient expresses a strong desire for non-invasive screening for fetal aneuploidy and asks what testing options can be performed at today's visit.

Which of the following is the most appropriate screening test to offer this patient today?

  1. Cell-free fetal DNA testingAnswer
  2. B
    First-trimester combined screening
  3. C
    Quadruple screening
  4. D
    Maternal serum alpha-fetoprotein testing
  5. E
    Chorionic villus sampling

Answer

Cell-free fetal DNA testing is the only appropriate non-invasive aneuploidy screening that can be performed at 14 weeks and 3 days gestation.
Cell-free fetal DNA (cfDNA) testing can be performed at any time after 10 weeks gestation and offers the highest sensitivity and specificity for common fetal aneuploidies among non-invasive screens. Because the patient is precisely 14 weeks and 3 days pregnant (based on the more accurate ultrasound dating overriding her LMP), she is in a unique gestational window. The first-trimester combined screening window closes at 13 weeks and 6 days, and the quadruple screening window does not open until 15 weeks and 0 days. Therefore, traditional serum analyte screenings are not applicable today, making cfDNA the only viable non-invasive option to offer.

Step-by-Step Solution

1
Determine the accurate gestational age.
The gestational age is 14 weeks and 3 days.
According to ACOG guidelines, when a first-trimester or early second-trimester ultrasound has a discrepancy of more than 7 days compared to the LMP dating (11 days in this case), the ultrasound dating must be used to establish the estimated due date.
2
Identify the clinical objective.
The patient desires a non-invasive screening test for fetal aneuploidy that can be drawn at today's visit.
This requirement restricts the choices to non-invasive tests with gestational age windows that include 14 weeks and 3 days.
3
Evaluate the gestational windows for available non-invasive screening options.
First-trimester combined screening is valid from 10w0d to 13w6d. Quadruple screening is valid from 15w0d to 22w6d. cfDNA can be performed anytime after 10w0d.
Accurate sequencing of prenatal care is essential to ensure tests are performed when their sensitivity and specificity are validated.
4
Select the appropriate test.
Cell-free fetal DNA testing is the only valid option.
The patient is in the 'gap' week (week 14) where neither the combined screen nor the quad screen can be performed, leaving cfDNA as the sole option.

Key Concept

Accurate pregnancy dating dictates the availability of aneuploidy screening tests; at 1414 weeks gestation, the first-trimester combined screen window has closed, and the quadruple screen window has not yet opened, leaving cell-free DNA (cfDNA) testing as the sole non-invasive option.
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Topics

Normal Pregnancy and Prenatal Care

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