What is the recommended evaluation and management approach for a patient at 11 weeks gestation?

Evaluation at 11 Weeks Gestation

At 11 weeks gestation, the primary evaluation should include a transabdominal ultrasound with nuchal translucency (NT) measurement as part of first-trimester combined aneuploidy screening, along with maternal serum markers (PAPP-A and free β-hCG), which provides detection rates exceeding 85% for major chromosomal abnormalities. 1

Core Components of the 11-Week Evaluation

Aneuploidy Screening

• First-trimester combined screening using NT measurement (11-14 weeks) plus maternal serum markers remains a reasonable and recommended option for all pregnant women, offering early reassurance and timely detection of abnormalities with the option for earlier and safer pregnancy termination if desired 1
• The American College of Obstetricians and Gynecologists recommends that aneuploidy screening or diagnostic testing be offered to all women in early pregnancy 1
• Risk estimates for trisomy 21,18, and 13 are calculated after adjusting for crown-rump length and maternal factors 1

Fetal Anatomic Assessment

• A detailed anatomic survey should be performed according to a standardized protocol at 11-13 weeks, which can identify many severe non-chromosomal fetal abnormalities 2
• Detection rates at 11-13 weeks are highest for:
  • Abdominal wall defects (100%) 3
  • Major central nervous system anomalies (88%) 3
  • Cardiac defects (74%) 3
  • Skeletal anomalies (71%) 3
• Approximately 70-79% of all major structural anomalies can be diagnosed in the first trimester 3, 4

Nuchal Translucency Measurement Standards

• NT measurement must be performed using standardized technique with quality assurance programs (Fetal Medicine Foundation or Nuchal Translucency Quality Review) to minimize inter- and intra-observer variability 1
• A threshold of ≥3 mm at 11-14 weeks defines increased NT and warrants further evaluation 1
• NT measurements are similar whether obtained transabdominally or transvaginally, as long as proper technique is applied 1

Management Based on NT Findings

Normal NT (<3 mm)

• No routine Doppler studies should be performed when NT is normal, given theoretical risk of thermal damage to the developing fetus from color and pulsed Doppler ultrasound 1
• No routine fetal echocardiography is indicated in the absence of other maternal or fetal risk factors 1
• A repeat examination at approximately 20 weeks gestation to complete screening for malformations is recommended 5

Increased NT (≥3 mm)

When NT measures ≥3 mm, the following cascade of evaluations is mandatory:

1. Immediate referrals:

   • Genetics consultation for counseling 1
   • Consideration of diagnostic genetic testing (chorionic villus sampling or amniocentesis) 1

2. Additional first-trimester markers:

   • Assessment for presence/absence of nasal bone (improves aneuploidy detection) 1
   • Evaluation of tricuspid valve flow and ductus venosus waveform using Doppler (reserved for NT ≥3 mm to minimize fetal exposure) 1
   • Significant tricuspid regurgitation or reversed a-waves in ductus venosus increase likelihood of aneuploidy 10-fold 1

3. Genetic testing recommendations:

   • Targeted genetic studies including microarray analysis are recommended in all cases of increased NT, as 10% of euploid fetuses with NT ≥3 mm have genetic variants consistent with Noonan syndrome 1
   • Cell-free fetal DNA testing alone is insufficient; diagnostic testing should be offered 1

4. Second-trimester follow-up (mandatory regardless of karyotype):

   • Detailed anatomic survey at 18-22 weeks 1
   • Fetal echocardiogram at 18-22 weeks (NT ≥3 mm is an accepted indication for fetal echo, with detection rate >80% for major cardiac anomalies) 1
   • These remain indicated even if fetal DNA analysis is normal, as structural anomaly risk persists in euploid fetuses 1

5. Third-trimester surveillance:

   • Serial fetal surveillance for growth and well-being is reasonable given increased risk of intrauterine demise, which is proportional to degree of NT enlargement 1

Special Considerations for Twin Pregnancies

Dichorionic Twins

• NT measurement allows individual assessment of each fetus using standard singleton thresholds 1
• Fetus-specific risks are calculated independently for each twin 1
• First-trimester combined screening provides detection rates >85%, similar to singletons 1
• Management of increased NT mirrors singleton approach: genetics referral, diagnostic testing consideration, and second-trimester anatomic survey plus fetal echocardiogram 1

Monochorionic Twins

• Pregnancy-specific risk is calculated using the mean NT thickness of both fetuses 1
• Detection rates are similar to singletons (87.4%) but with higher false-positive rates 1
• Increased NT may indicate early twin-twin transfusion syndrome (TTTS) with hypervolemic congestion in recipient twin 1
• NT discordance >20% is associated with >30% risk of severe TTTS and early fetal death 1
• All monochorionic twins require serial ultrasound surveillance every 2 weeks starting at 16 weeks regardless of NT measurements, due to complications from shared placentation 1
• Second-trimester fetal echocardiography is warranted in all monochorionic twins due to substantial risk of structural and functional heart abnormalities 1

Common Pitfalls to Avoid

• Do not perform NT screening solely for aneuploidy detection if cell-free fetal DNA testing has already been performed and is negative, as this provides limited additional value 1
• Do not skip second-trimester anatomic survey and fetal echocardiogram in cases of increased NT even when karyotype is normal, as 70% of structural anomalies in euploid fetuses with normal NT are still diagnosed in the first trimester, but comprehensive evaluation requires second-trimester imaging 1
• Do not use Doppler studies routinely in low-risk pregnancies with normal NT due to theoretical thermal risks to the developing fetus 1
• Do not rely on first-trimester scan alone for complete anomaly detection, as common abnormalities like ventriculomegaly, isolated cleft lip, and ventricular septal defects are detected in <10% of cases at 11-13 weeks 2

Documentation and Follow-up

• Ensure proper documentation of crown-rump length for accurate gestational age dating 1
• Schedule 18-20 week anatomy scan for all patients, as this remains the optimal time for comprehensive structural evaluation 6, 7
• For high-risk findings, coordinate multidisciplinary care including maternal-fetal medicine, genetics, and pediatric subspecialists as appropriate 8