What is the purpose and procedure of an obstetric (OB) ultrasound scan at approximately 19-20 weeks of gestation?

Obstetric Ultrasound at 19-20 Weeks Gestation

Primary Purpose and Timing

The obstetric ultrasound scan at approximately 19-20 weeks (1.5 periods of gestation) is the standard anatomical survey to detect fetal structural abnormalities, and should be performed between 18-22 weeks' gestation in all pregnant women. 1, 2, 3

This timing represents the optimal window when fetal anatomy is sufficiently developed for visualization while still allowing time for clinical decision-making if abnormalities are detected. 1, 2

What This Scan Evaluates

Core Anatomical Assessment

The examination systematically evaluates:

• Central nervous system: Brain structures, ventricles, posterior fossa, spine for neural tube defects 1
• Craniofacial structures: Skull mineralization, facial profile, cleft lip/palate 1
• Cardiac anatomy: Four-chamber view, outflow tracts, cardiac position 1
• Thoracic structures: Lungs, diaphragm, evaluation for congenital diaphragmatic hernia or pulmonary airway malformations 1
• Abdominal organs: Stomach, kidneys, bladder, abdominal wall integrity (gastroschisis, omphalocele) 1
• Skeletal system: Long bones, spine, extremities for skeletal dysplasias 1
• Placenta and cord: Placental location, cord insertion site, amniotic fluid volume 1

Additional Assessments

• Gestational age confirmation: Using composite measurements (biparietal diameter, head circumference, abdominal circumference, femur length) accurate to within 10 days 2
• Fetal growth parameters: Estimated fetal weight 1
• Soft markers: Minor findings that may indicate increased aneuploidy risk (echogenic intracardiac focus, choroid plexus cysts, renal pyelectasis, shortened long bones, nuchal thickening, echogenic bowel) 1

Clinical Impact and Detection Rates

The 19-week anatomical scan detects approximately 85% of major fetal structural abnormalities with 99.9% specificity. 4 For the pre-selected group of eight major anomalies, first-trimester screening shows 93.3% detection, though cardiac anomalies are detected at lower rates (55.8%) even with specialized evaluation. 5

Procedure Details

Standard Approach

• Primary method: Transabdominal ultrasound 1
• Examination time: 20-30 minutes for complete anatomical survey 6
• Supplementary imaging: Transvaginal ultrasound may be used when transabdominal visualization is suboptimal, particularly for detailed pelvic or cervical assessment 1, 7

Special Population Considerations

Obese patients (BMI ≥35 kg/m²) require modified timing:

• Schedule the anatomical survey at 20-22 weeks (approximately 2 weeks later than standard timing) due to decreased visualization through increased body habitus 1, 2, 3
• If the initial scan is incomplete, schedule repeat follow-up in 2-4 weeks 1, 2, 3
• Consider early transvaginal anatomic evaluation at 12-16 weeks combined with routine transabdominal study at 18-22 weeks to achieve completion rates comparable to non-obese populations 1, 7

Multiple gestations require additional evaluation:

• Chorionicity and amnionicity determination (ideally established earlier in first trimester) 1, 7
• Individual fetal anatomical surveys for each twin 1
• Placental cord insertion sites for each fetus (velamentous insertion occurs in up to 22% of monochorionic twins and increases risk of complications) 1
• Umbilical artery Doppler assessment for twin-twin transfusion syndrome staging 1
• Fetal echocardiography recommended due to 2% risk in uncomplicated monochorionic twins, 5% in twin-twin transfusion syndrome cases 1

Follow-Up Based on Findings

When Detailed Scan is Indicated

If the routine anatomy scan identifies:

• Major anomalies: Immediate detailed ultrasound examination by maternal-fetal medicine specialist 1
• Soft markers with additional findings: Detailed scan to evaluate for associated anomalies 1
• Incomplete examination: Repeat scan in 2-4 weeks 1

When MRI is Appropriate

Fetal MRI (without contrast) is indicated when:

• Ultrasound findings are incomplete or inconclusive, particularly for central nervous system anomalies 1
• Optimal timing is at or after 22 weeks' gestation, though 18-22 weeks may provide value in specific clinical scenarios 1
• Particularly helpful for: isolated ventriculomegaly, agenesis of corpus callosum, absent cavum septi pellucidi, cerebellar/vermian anomalies, neck masses requiring airway planning 1

When Fetal Echocardiography is Needed

Specialized cardiac evaluation is indicated for:

• Abnormal cardiac screening on anatomy scan 1
• Maternal risk factors: pregestational diabetes, teratogen exposure, family history of cardiac disease 1
• Multiple gestations (especially monochorionic) 1
• Other fetal anomalies detected (cardiac defects often occur with other structural abnormalities) 1

Common Pitfalls to Avoid

• Delaying beyond 22 weeks: This limits clinical options if severe abnormalities requiring termination are detected 2
• Using standard timing in obese patients: Results in suboptimal visualization and missed anomalies; delay to 20-22 weeks 1, 2, 3
• Performing unnecessary follow-up for isolated soft markers: When aneuploidy screening (NIPT, first-trimester combined screening) is negative, isolated echogenic intracardiac focus or choroid plexus cysts do not require detailed follow-up 3
• Relying solely on last menstrual period dating: Ultrasound composite measurements at this scan are more accurate and should be used to establish or confirm gestational age 2
• Missing velamentous cord insertion in twins: This finding in monochorionic pregnancies significantly increases risk of twin-twin transfusion syndrome (27% vs 7% with normal insertions) and requires closer surveillance 1