At what gestational ages in weeks are ultrasounds typically recommended for a standard, low-risk pregnancy?

Standard Ultrasound Timing in Low-Risk Pregnancy

For a standard, low-risk pregnancy, you should obtain an anatomy scan at 18-20 weeks' gestation, with consideration for an additional growth scan at 36 weeks' gestation. 1

First Trimester (11-14 weeks)

• Dating ultrasound should be performed in the first trimester when possible, as ultrasound dating is more accurate than last menstrual period dating and improves screening sensitivity and specificity 2
• First-trimester scans have 91.3% sensitivity for detecting lethal anomalies but only 37.5% sensitivity for all structural anomalies, making them useful for early detection of severe abnormalities but insufficient as standalone screening 3
• Nuchal translucency measurement at 10-14 weeks can identify fetuses at risk for severe skeletal dysplasia or chromosomal abnormalities, prompting follow-up evaluation 1

Second Trimester (18-20 weeks)

• The anatomical survey at 18-20 weeks is the cornerstone of prenatal ultrasound screening, with multi-specialty consensus (ACR-ACOG-AIUM-SMFM-SRU) recommending at least one ultrasound be offered to all pregnant women during this window 2, 1
• This scan evaluates fetal structure and development with optimal visualization for most organ systems 1
• Two-stage screening (first trimester + second trimester combined) detects 83.8% of structural anomalies before 24 weeks, compared to only 50.5% with single-stage screening at 18-20 weeks alone 3
• The scan remains acceptable through 20 weeks 6 days, though screening performance may decline after the optimal 18-20 week window 2

Third Trimester (36 weeks)

• Routine third-trimester ultrasound at 36 weeks is NOT recommended for low-risk women according to 83% of international guidelines 4, 1
• However, recent evidence suggests a 36-week scan may be superior to 32-week scanning for detecting late-onset fetal growth restriction, with sensitivity of 48% vs 22.5% respectively 5, 6
• A 2024 pre-post intervention study showed that universal third-trimester ultrasound at 36.0-37.6 weeks in low-risk pregnancies demonstrated 83% probability of reducing composite neonatal adverse outcomes, though the number needed to treat was 121 7

Indications for Additional Scans

Perform ultrasound when:

• Fundal height measurement is >3 cm less than gestational age in weeks 4
• Obesity (BMI ≥35 kg/m²) or fibroid uterus makes fundal height measurements unreliable 4, 1
• Any clinical suspicion of abnormal fetal growth arises during routine prenatal care 4

Special Timing Modifications

For obese patients (BMI ≥35 kg/m²):

• Schedule the anatomy scan at 20-22 weeks (approximately 2 weeks later than normal-weight women) due to suboptimal visualization 2, 1
• If the anatomical survey is incomplete, schedule repeat follow-up ultrasound in 2-4 weeks 2
• Consider a growth scan at 28-32 weeks to aid detection of late-onset fetal growth restriction when clinical assessment is limited 1

Common Pitfalls to Avoid

• Do not delay the anatomy scan beyond 22 weeks, as this limits clinical options if abnormalities are detected 2
• Do not perform unnecessary follow-up scans for isolated soft markers (like echogenic intracardiac focus or choroid plexus cysts) when aneuploidy screening is negative 1
• Do not rely solely on LMP dating - use composite ultrasound measurements from the anatomical scan to establish accurate gestational age, as second-trimester ultrasound dating based on multiple parameters is accurate to within 10 days 2
• Avoid routine third-trimester screening in truly low-risk populations, as it has not shown evidence of improved antenatal, obstetric, or neonatal outcomes in most studies, though emerging data suggests potential benefit 4, 1, 7