If a patient missed dating/first trimester ultrasound (US) and is now 20+3 weeks gestational age based on last menstrual period (LMP), should we wait for anatomical/second-trimester ultrasound and if so, at what gestational age should it be performed?

Anatomical Ultrasound Timing for Missed First Trimester Scan

At 20+3 weeks gestational age based on LMP, you should proceed immediately with the anatomical ultrasound rather than waiting, as you are already within the optimal window of 18-22 weeks for second-trimester anatomical screening. 1

Recommended Timing and Approach

Perform the anatomical ultrasound now (at 20+3 weeks) because:

• The standard anatomical survey should be performed between 18-22 weeks' gestation, with multi-specialty consensus recommending at least one ultrasound be offered to all pregnant women during this window 1
• At 20+3 weeks, you are already past the midpoint of the optimal screening window, making immediate scheduling appropriate rather than further delay 1
• The ACR Appropriateness Criteria specifically state that components of the standard fetal examination at 18-20 weeks have been agreed upon by multiple organizations (ACR-ACOG-AIUM-SMFM-SRU) 1

Critical Dating Consideration

Because this patient has no prior ultrasound for dating, the anatomical scan must also establish accurate gestational age:

• Ultrasound dating is more accurate than LMP dating and improves both sensitivity and specificity of screening 1
• In the second trimester, ultrasound dating based on composite measurements (multiple parameters) is accurate to within 10 days and is preferable to LMP dating 1
• For pregnancies at 14 weeks or later when LMP is uncertain, biparietal diameter (BPD) measurement is specifically recommended as it can rule out anencephaly and improve detection of open spina bifida 1
• The anatomical scan will provide composite ultrasound dating that should be used to adjust the estimated due date if there is significant discrepancy with LMP dates 1

Practical Timing Considerations

If the patient cannot be scheduled immediately:

• The anatomical survey remains acceptable through 20.9 weeks (20 weeks 6 days), though screening performance may begin to decline after the optimal 18-20 week window 1
• Research demonstrates that scans at 20-22 weeks have significantly higher completion rates (88-90%) compared to 18 weeks (76%), with fewer rescans needed 2
• However, waiting beyond 22 weeks is not advisable as it limits clinical options if abnormalities are detected 1

Special Populations Requiring Modified Timing

For obese patients (BMI ≥35 kg/m²):

• Consider scheduling at 20-22 weeks (approximately 2 weeks later than normal weight women) due to suboptimal visualization 1
• If the anatomical survey is incomplete, schedule a repeat follow-up ultrasound in 2-4 weeks 1
• Early transvaginal ultrasound at 13-15 weeks combined with routine transabdominal study at 18-22 weeks can improve completion rates to match non-obese populations 1, 3

Common Pitfalls to Avoid

• Do not delay the scan beyond 22 weeks thinking you need to wait for a specific "anatomical scan appointment" - the patient is already in the optimal window 1
• Do not rely solely on LMP dating for this pregnancy - use the composite ultrasound measurements from the anatomical scan to establish accurate gestational age 1
• Do not assume a single scan will be sufficient - approximately 10-24% of patients may require a follow-up scan to complete the anatomical survey, particularly if maternal body habitus limits visualization 2