Recommended Frequency of Ultrasound Scans During Pregnancy
At least one ultrasound scan should be offered routinely to all pregnant women between 18 and 20 weeks of gestation, with additional scans based on risk factors and clinical findings. 1
Standard Ultrasound Schedule for Low-Risk Pregnancies
- First trimester (11-14 weeks): Dating scan and nuchal translucency measurement for aneuploidy screening 1, 2
- Mid-trimester (18-20 weeks): Standard anatomy scan to evaluate fetal structure and development 1
- Third trimester: Selective use based on clinical indications rather than routine screening for all pregnancies 1
Indications for Additional Ultrasound Scans
First Trimester Considerations
- Increased nuchal translucency at 10-14 weeks may indicate severe skeletal dysplasia or chromosomal abnormalities, requiring follow-up evaluation 1, 3
- Transvaginal ultrasound may supplement transabdominal scans when fetal structures are difficult to visualize, particularly in obese patients 1
Follow-up Scans for Specific Findings
- For isolated echogenic bowel: Third-trimester ultrasound for reassessment and evaluation of growth 1
- For isolated single umbilical artery: Third-trimester ultrasound for growth evaluation with consideration of weekly antenatal surveillance beginning at 36 weeks 1
- For isolated urinary tract dilation A1: Follow-up ultrasound at 32 weeks 1
- For isolated urinary tract dilation A2-3: Individualized follow-up schedule with planned postnatal evaluation 1
- For isolated shortened humerus, femur, or both: Third-trimester ultrasound for reassessment and growth evaluation 1
No Additional Scans Needed
- For isolated echogenic intracardiac focus or isolated choroid plexus cysts with negative serum or cell-free DNA screening: No further ultrasound evaluation needed as these are normal variants 1, 4
High-Risk Pregnancy Considerations
Multiple gestations require more frequent monitoring than singleton pregnancies 1
- First trimester dating scan
- Nuchal translucency scan at 11-14 weeks
- Anatomy scan at 18-22 weeks
- One or more scans in the third trimester for growth assessment 1
Monochorionic twin pregnancies require more intensive surveillance due to higher complication rates 1
Maternal obesity may require:
- Delayed anatomy scan at 20-22 weeks (about 2 weeks later than for women of normal weight)
- Repeat follow-up in 2-4 weeks if initial scan is incomplete 1
Evidence Quality and Clinical Implications
First-trimester detailed scans can detect approximately 50-70% of major structural abnormalities, but cannot replace the standard mid-trimester scan 2, 5, 6
Third-trimester routine screening has not shown evidence of improved antenatal, obstetric, or neonatal outcomes in low-risk pregnancies, but may have utility for perinatal management in specific cases 1
The detection rate for structural abnormalities is approximately 59% in early pregnancy and increases to 81% when combined with second-trimester scanning 7
Common Pitfalls to Avoid
- Relying solely on first-trimester scans for anomaly detection, as many structural abnormalities become apparent only in the second trimester 2, 6
- Performing unnecessary follow-up scans for isolated soft markers (like echogenic intracardiac focus or choroid plexus cysts) when aneuploidy screening is negative 1, 4
- Failing to adjust timing of anatomy scans for maternal obesity, which can lead to suboptimal visualization and missed anomalies 1