What are the next steps in prenatal care with a fetal crown-rump length (CRL) of 43.92mm on ultrasound?

Gestational Age and Next Steps for CRL 43.92mm

With a crown-rump length (CRL) of 43.92mm, this pregnancy is approximately 11 weeks 5 days gestational age, and the next steps include confirming cardiac activity, scheduling nuchal translucency (NT) screening between 11-14 weeks if not already performed, and establishing accurate pregnancy dating for all subsequent prenatal care decisions. 1, 2

Gestational Age Determination

• A CRL of 43.92mm corresponds to approximately 11 weeks 5 days gestational age, placing this pregnancy within the optimal window for first-trimester screening 1, 3
• This CRL measurement should be used as the definitive gestational age for all clinical decision-making, regardless of last menstrual period (LMP) dating, as first-trimester CRL provides accuracy within ±5-7 days 2
• Document both the CRL-based and LMP-based dates in the medical record, but use the CRL-based date (11 weeks 5 days) for all clinical decisions 2

Immediate Assessment Requirements

Cardiac Activity Verification

• Confirm presence of cardiac activity, as this is mandatory for any embryo with CRL ≥7mm 1
• Document cardiac activity with M-mode or cine clip, and record the heart rate in beats per minute 1
• Absence of cardiac activity with a CRL of 43.92mm would be diagnostic of embryonic/fetal demise and require immediate counseling 1

Viability Assessment

• At this CRL (43.92mm), the pregnancy has transitioned from embryonic to fetal stage (occurs at approximately 10 weeks gestational age) 1
• Evaluate for poor prognostic indicators including bradycardia, small gestational sac relative to embryo size, enlarged amniotic cavity (expanded amnion sign), or abnormal yolk sac appearance 1

First-Trimester Screening Window

Nuchal Translucency Screening

• This pregnancy is within the optimal window for NT screening, which should be performed between 11 weeks 0 days and 13 weeks 6 days 1
• The acceptable CRL range for NT screening is 45-84mm according to standard protocols 1
• Since this CRL is 43.92mm (just below 45mm), schedule NT screening within the next few days as the fetus grows approximately 1mm per day 1
• NT screening requires specialized sonographer training and credentialing, so ensure the examination is performed by qualified personnel 1

Combined First-Trimester Screening

• NT measurement combined with maternal serum biochemistry (PAPP-A and free β-hCG) provides optimal Down syndrome screening in the first trimester 1
• Accurate CRL measurement is critical for combined screening, as even a 5mm measurement error can significantly alter risk calculations and lead to misclassification in up to 27.5% of borderline cases 4

Multiple Gestation Considerations

• If not already determined, confirm whether this is a singleton or multiple gestation pregnancy 1
• For twin pregnancies at this gestational age, determine chorionicity and amnionicity, as this is crucial for subsequent management 1
• In multiple gestations, use the larger twin's CRL for dating purposes to minimize the chance of missing growth restriction 1
• Significant CRL discordance between twins (>20%) increases risk of subsequent demise and warrants closer surveillance 1

Documentation and Quality Assurance

Required Documentation

• Record the CRL measurement in millimeters (43.92mm), gestational age in weeks and days (11 weeks 5 days), and number of fetuses 1
• Express gestational age as weeks and days rather than rounded weeks for optimal accuracy 2
• Identify the sonographer performing the measurement using a unique code (not initials) 1

Measurement Technique Verification

• Ensure CRL was measured as the greatest length from crown to rump, excluding limbs and yolk sac 2
• Verify the measurement was obtained with proper technique, as quality control is essential for accurate dating and screening 1, 4

Clinical Implications for Ongoing Care

Timing of Subsequent Assessments

• Schedule the detailed fetal anatomy scan at 18-22 weeks based on this CRL-derived gestational age 2
• Plan maternal serum AFP screening (if performing quad screen) at 15-20 weeks, recognizing that AFP levels increase 10-15% per week and require precise dating 2
• Never revise the gestational age based on later ultrasounds unless there is a discrepancy of ≥10 days, as first-trimester CRL dating remains the gold standard 2

Risk Stratification

• A CRL deficit of >2 standard deviations below the mean for gestational age in threatened miscarriage cases increases risk of subsequent pregnancy loss or small-for-gestational-age infants 5
• At this CRL (43.92mm), the expected mean CRL for 11 weeks 5 days should be verified against established curves to identify any growth concerns 3, 6

Common Pitfalls to Avoid

• Never average LMP and ultrasound dates - always use the CRL-based date when obtained in the first trimester 2
• Do not delay NT screening beyond 13 weeks 6 days, as the window will close and the opportunity for first-trimester aneuploidy screening will be lost 1
• Avoid using terms like "heartbeat" or "viable" in first-trimester documentation; instead use "cardiac activity" and "cardiac motion" 1
• Do not reclassify screening results from positive to negative based on dating changes unless the revised gestational age differs by at least 10 days 2