What are the pregnancy screenings by week of gestation?

Comprehensive Pregnancy Screening Schedule by Gestational Week

Pregnancy screening should follow a systematic timeline with specific tests at different gestational ages to optimize detection of fetal abnormalities, maternal conditions, and ensure optimal maternal and fetal outcomes.

First Trimester Screenings (Weeks 1-13)

Weeks 6-8: Initial Prenatal Visit

• Complete blood count
• Blood type and Rh factor
• Antibody screen
• Rubella immunity status
• Hepatitis B surface antigen
• HIV testing
• Syphilis screening
• Chlamydia and gonorrhea testing
• Urine culture and analysis
• Pap smear (if due)

Weeks 11-13: First Trimester Screening

• First trimester Down syndrome screening (11-13 completed weeks)
  • Nuchal translucency (NT) ultrasound measurement
  • Maternal serum PAPP-A (pregnancy-associated plasma protein A)
  • Free beta-hCG (human chorionic gonadotropin) or total/intact hCG 1
  • Crown-rump length (CRL) measurement for accurate gestational dating 1

Clinical sensitivity and specificity for Down syndrome screening are best at 11 completed weeks and reduced by 13 completed weeks' gestation 1.

Second Trimester Screenings (Weeks 14-27)

Weeks 15-20: Second Trimester Screening

• Maternal serum screening for open neural tube defects (ONTD) (optimal at 16-18 weeks, acceptable between 15.0-20.9 weeks) 1

  • Alpha-fetoprotein (AFP) measurement
  • Adjustments made for maternal weight, race, and insulin-dependent diabetes mellitus 1

• Detailed anatomy ultrasound (18-22 weeks)

  • Comprehensive evaluation of fetal anatomy
  • Assessment for structural abnormalities
  • Placental location evaluation

Week 24-28: Gestational Diabetes Screening

• Glucose challenge test (GCT) or Glucose tolerance test (GTT)

  • USPSTF recommends screening for gestational diabetes after 24 weeks of gestation (B recommendation) 1
  • Most commonly used screening test is the 50-g oral glucose challenge test (OGCT) 1

• Additional second trimester screenings:

  • Repeat complete blood count
  • Antibody screen for Rh-negative mothers
  • Administration of Rh immunoglobulin if indicated

Third Trimester Screenings (Weeks 28-40)

Weeks 28-32:

• Group B Streptococcus (GBS) screening
• Repeat blood count to check for anemia
• Repeat glucose screening if indicated

Weeks 32-36:

• Fetal presentation assessment
• Fetal growth assessment
• Repeat HIV and syphilis testing in high-risk populations

Weeks 36-40:

• Weekly cervical examinations (if indicated)
• Fetal well-being assessment
• Non-stress test or biophysical profile (if indicated)

Important Considerations for Screening Implementation

Timing Considerations

1. First trimester screening: Best performed at 11 completed weeks, with reduced effectiveness by 13 completed weeks 1
2. ONTD screening: Optimal between 16-18 weeks, acceptable 15.0-20.9 weeks 1
3. Gestational diabetes: After 24 weeks of gestation 1

Factors Affecting Screening Accuracy

• Maternal weight: Affects levels of screening markers; heavier women have lower serum marker levels 1
• Maternal race: AFP levels in Black/African American women are 10-15% higher than in Caucasian women 1
• Maternal insulin-dependent diabetes: AFP levels 10-20% lower in women with IDDM 1
• Gestational age accuracy: Best expressed as weeks and days or decimal weeks rather than rounded weeks 1

Dating Methods

• First trimester: Crown-rump length (CRL) provides accurate gestational age estimation to within 7 days 1
• Second trimester: Multiple measurements (composite) ultrasound dating accurate to within 10 days; BPD dating recommended for ONTD screening after 14 weeks 1

Modern Screening Approaches

Two main approaches to prenatal screening for structural abnormalities:

1. Single-stage screening: One ultrasound in second trimester
2. Two-stage screening: Combined first and second trimester ultrasounds 2

Two-stage screening shows higher sensitivity (83.8%) compared to single-stage screening (50.5%) for detecting structural abnormalities 2.

First-trimester ultrasound has high sensitivity (91.3%) for detecting lethal anomalies despite limited overall sensitivity (37.5%) for all structural abnormalities 2.

Evolving Recommendations

Recent advances suggest that cell-free DNA testing should be considered for chromosomal abnormality screening due to its high sensitivity (>99% for Down syndrome) 3.

Chromosomal microarray analysis (CMA) should be considered when invasive testing is performed, as it can detect clinically important chromosomal aberrations not detected by standard karyotype 3.

By following this comprehensive screening schedule, healthcare providers can optimize detection of fetal abnormalities and maternal conditions to improve pregnancy outcomes.