What are the recommended maternal serum screening requirements for pregnant women, including timing and specific serum markers?

Maternal Serum Screening Requirements

All pregnant women should be offered maternal serum alpha-fetoprotein (MSAFP) screening optimally between 16-18 weeks gestation for detection of open neural tube defects and anencephaly, and multiple marker screening (AFP, hCG, uE3 with or without inhibin A) should be offered for aneuploidy detection at 15-20 weeks gestation unless amniocentesis is already indicated or first trimester screening has been performed. 1

Second Trimester Screening Protocol

Timing and Gestational Age Requirements

• The multiple marker screening (MMS) test must be performed between 15-20 weeks of gestation, with optimal timing at 16-18 weeks. 1
• The laboratory must be informed of accurate gestational age at the time of sample collection for proper interpretation. 1
• If gestational age changes by 2 or more weeks after ultrasound examination, test results must be reinterpreted. 1
• If the sample was drawn at less than 15 weeks, a new sample should be sent with corrected gestational age. 1

Serum Markers for Aneuploidy Screening

The quad screen (AFP, hCG, unconjugated estriol [uE3], and inhibin A) achieves the highest detection rate for Down syndrome in the second trimester at approximately 80-81% with a 5% false positive rate. 2

The following marker combinations are available:

• Triple screen: AFP, unconjugated estriol, hCG (detection rate ~69%) 1
• Quad screen: AFP, unconjugated estriol, hCG, inhibin A (detection rate ~81%) 1, 2
• Multiple marker screening should be offered to all women unless amniocentesis is indicated based on history/age or first trimester screening was elected. 1

Neural Tube Defect Screening

• MSAFP screening for open neural tube defects and anencephaly should be offered to ALL women, including those who had first trimester screening or CVS. 1
• This screening remains essential even after first trimester aneuploidy screening because first trimester tests do not detect neural tube defects. 3

First Trimester Screening Options

Combined First Trimester Screening

First trimester screening using nuchal translucency (NT), pregnancy-associated plasma protein A (PAPP-A), and free beta-hCG can be performed at 11-14 weeks gestation with detection rates comparable to second trimester quad screening. 1

• NT measurement combined with PAPP-A and free beta-hCG achieves approximately 82-87% detection rate for Down syndrome with 5% false positive rate. 1, 3
• First trimester screening requires strict adherence to quality standards and appropriate ultrasound training. 1, 3
• Sonographers must be appropriately trained in proper NT measurement technique and maintain appropriate certification. 3

Critical NT Findings

• An NT measurement of 3.5 mm or greater, even with low risk screening result or normal fetal karyotype, should prompt detailed anatomic ultrasound and/or echocardiogram. 3
• Increased NT is associated not only with aneuploidy but also with congenital heart defects, diaphragmatic hernias, skeletal dysplasias, and genetic syndromes. 3

Integrated and Sequential Screening Approaches

Integrated screening (combining first trimester NT and PAPP-A with second trimester AFP, hCG, uE3, and inhibin A) provides the most sensitive and cost-effective approach with detection rates of 94-96%. 1

• Integrated screening yields one final interpretation after both trimesters are complete. 1
• Sequential screening discloses first trimester results and offers testing if positive, with second trimester screening if first trimester is negative (detection rate ~95%). 1
• Providers must ensure adequate follow-up and confirm that nondisclosure is acceptable to patients if offering integrated screening. 3

Special Populations and Considerations

Advanced Maternal Age

• CVS and amniocentesis should continue to be offered to women 35 years and older for diagnosis of aneuploidy. 1
• Age-based cutoffs are no longer the sole criterion for offering invasive testing; women should be informed of adjusted risk and allowed to make decisions based on individual risk assessment. 3

Multiple Gestations

• First trimester screening can be used in multifetal pregnancies, but women must be informed of the limitations in this setting. 3
• Screening performance is diminished in twin pregnancies compared to singletons. 4

Counseling Requirements

Genetic counseling and/or educational materials must be available to review different screening tests, including information about Down syndrome, trisomy 18, and open neural tube defects, along with benefits, risks, limitations, and possible outcomes. 1

• Patients who are screen positive for trisomy 21 or 18 should be offered genetic counseling and amniocentesis. 1
• Women should be adequately counseled regarding advantages and limitations of screening tests and availability of prenatal diagnostic tests. 1

Quality Control Requirements

Laboratory Standards

• Accurate gestational age determination is critical for proper interpretation. 1
• First day of last menstrual period is the most common method, but ultrasound measurement of crown-rump length (first trimester) or biparietal diameter (second trimester) provides more accurate estimates within 7 and 10 days, respectively. 1

Ultrasound Standards

• First trimester screening requires appropriate ultrasound training and quality control before implementation in routine practice. 1
• Long-term monitoring of both laboratory and ultrasound measures should be part of internal and external quality assurance programs. 5

Common Pitfalls to Avoid

• Do not skip MSAFP screening in women who had first trimester screening—neural tube defects cannot be detected by first trimester serum markers alone. 1, 3
• Do not ignore elevated NT measurements even with normal karyotype—these require additional anatomic evaluation for structural anomalies. 3
• Do not fail to reinterpret results if gestational age changes by ≥2 weeks—this significantly affects risk calculations. 1
• Ensure proper training and certification for NT measurements, as technique significantly impacts screening performance. 3