What are the guidelines for antenatal scans in pregnant women?

Antenatal Ultrasound Scanning Guidelines

All pregnant women should receive at least one ultrasound scan between 18-22 weeks gestation for anatomical assessment, with additional scans at 11-14 weeks for dating/aneuploidy screening and in the third trimester (28-32 weeks) for growth assessment in high-risk pregnancies. 1, 2

Standard Timing and Indications for Routine Scans

First Trimester (11-14 weeks)

• Dating ultrasound should be performed in the first trimester to establish accurate gestational age, which is critical for all subsequent pregnancy management decisions 1
• Nuchal translucency measurement must be performed between 11 weeks 4 days and 13 weeks 6 days as part of first-trimester combined aneuploidy screening, achieving 85-92% detection rates for Down syndrome 1
• Assessment of chorionicity and amnionicity is mandatory for multiple pregnancies at this visit 1
• First-trimester anatomical screening can detect approximately 70% of major fetal anomalies, with an overall antenatal detection rate of 95% when combined with second-trimester scanning 3

Second Trimester (18-22 weeks)

• The primary anatomical survey should occur between 18-22 weeks gestation, as this represents the optimal window for visualizing fetal structures 2
• This scan screens for major structural abnormalities affecting all organ systems 4
• For women with BMI >40 kg/m², consider an early anatomy scan between 14-16 weeks to reduce problems with impaired acoustic windows, followed by the routine 20-22 week morphology scan 2

Third Trimester (28-32 weeks)

• Growth scans at 28-32 weeks are indicated for high-risk pregnancies where clinical assessment may be limited or specific risk factors exist 2
• Serial growth scans are recommended for women with ongoing substance use, tobacco use, or opioid use disorder due to increased risk of intrauterine growth restriction 2
• At least one fetal growth scan in the third trimester is recommended for women with opioid use disorder because of associations with low birth weight and small-for-gestational-age infants 2

High-Risk Pregnancy Surveillance

Indications for Additional Scanning

Antenatal fetal testing and additional ultrasound surveillance should be reserved exclusively for high-risk pregnancies, as testing in low-risk pregnancies causes more harm than benefit through false-positive results leading to unnecessary interventions 5, 2

High-risk conditions warranting additional surveillance include:

• Maternal conditions: Hypertensive disorders, diabetes mellitus, chronic renal disease, obesity (BMI ≥35 kg/m²) 5, 1
• Fetal conditions: Intrauterine growth restriction, structural anomalies, genetic syndromes 5
• Obstetric history: Previous stillbirth, previous preeclampsia, advanced maternal age (>35 years) 1

Timing of Surveillance Initiation

• Initiate antenatal testing at 32-34 weeks for most high-risk conditions, but adjust based on specific risk factors and likelihood of neonatal survival with intervention 5, 2
• Earlier initiation may be warranted for severe maternal disease or significant fetal compromise 2

Testing Frequency

• Weekly or twice-weekly testing has become standard practice in high-risk pregnancies, though this frequency should be balanced against the psychological burden of frequent visits 5, 2
• All standard tests (biophysical profile, modified biophysical profile) have high negative predictive values (>99.9%) for stillbirth within one week of a normal test 5

Special Populations Requiring Modified Protocols

Women with Obesity (BMI ≥30 kg/m²)

• Nuchal scan between 11 weeks 4 days and 13 weeks 6 days for women with BMI >40 kg/m² 2
• Early anatomy assessment at 14-16 weeks gestation to overcome acoustic window limitations 2
• Routine morphology scan at 20-22 weeks 2
• Growth scan at 28-32 weeks where clinical assessment is limited by obesity 2
• Obesity alone does not constitute an indication for routine antenatal fetal surveillance beyond these structural assessments 2

Women with Opioid Use Disorder

• At least one third-trimester growth scan is recommended due to increased risk of growth restriction 2
• Some providers recommend serial growth scans with ongoing illicit substance or tobacco use 2
• Antenatal testing with non-stress testing or biophysical profiles may be considered with ongoing opioid use, though data supporting this practice are limited 2

Women with History of Bariatric Surgery

• Maintain high index of suspicion for fetal growth abnormalities 2
• Fetal growth monitoring during pregnancy is important, though specific scanning protocols are not defined 2

Critical Limitations and Counseling Points

What Ultrasound Cannot Detect

• Routine antenatal testing cannot predict stillbirth related to acute changes in maternal-fetal status such as placental abruption or cord accidents 5, 2
• Up to half of all stillbirths occur in patients without recognized risk factors, meaning even comprehensive testing cannot eliminate all risk 5

Managing Patient Expectations

• Emphasize the extremely high negative predictive value (>99.9%) of normal testing to provide strong reassurance and reduce anxiety between tests 5
• Clearly explain to low-risk patients that excessive testing causes more harm than benefit through false-positive results 5
• Patient education and doctor-patient communication are the two most important aspects to enhance antenatal ultrasound quality 6

Response to Abnormal Findings

• When test results are abnormal before term, weigh the risks of prematurity against the risks of intrauterine death rather than proceeding immediately to delivery 5
• Consider repeat testing or alternative tests for equivocal results 5
• At term gestation, delivery is warranted for abnormal antenatal testing, as the risks of continued pregnancy outweigh the minimal risks of term delivery 5