What is the typical routine prenatal ultrasound schedule?

Routine Prenatal Ultrasound Schedule

For uncomplicated singleton pregnancies, the standard schedule includes a first-trimester dating ultrasound (ideally before 14 weeks) and an anatomy scan at 18-20 weeks, with additional scans reserved for specific clinical indications rather than routine third-trimester screening. 1, 2

Standard Singleton Pregnancy Schedule

First Trimester (Before 14 weeks)

• Dating ultrasound should be performed as early as possible to establish accurate gestational age, which is more reliable than last menstrual period dating 2, 3
• First-trimester scans improve detection of multiple pregnancies and reduce maternal anxiety about pregnancy 3, 4
• Nuchal translucency screening can be offered between 11-14 weeks for aneuploidy risk assessment 5, 6

Second Trimester (18-20 weeks)

• The anatomy scan between 18-20 weeks is the cornerstone of routine prenatal ultrasound, recommended by multi-specialty consensus (ACR-ACOG-AIUM-SMFM-SRU) 1, 2
• This scan evaluates fetal structure, development, and screens for major anomalies 1, 4
• The optimal window extends through 20 weeks 6 days, though performance may decline after 20 weeks 2
• For obese patients (BMI ≥35 kg/m²), schedule at 20-22 weeks due to suboptimal visualization, with repeat follow-up in 2-4 weeks if incomplete 5, 1, 2

Third Trimester

• Routine third-trimester ultrasound is NOT recommended for low-risk pregnancies, as it has not shown evidence of improved antenatal, obstetric, or neonatal outcomes 1
• Third-trimester scans are reserved for specific indications such as growth concerns, clinical complications, or high-risk conditions 1

High-Risk Pregnancy Modifications

Multiple Gestations

Multiple pregnancies require substantially more frequent monitoring than the standard schedule 5, 6:

Dichorionic Twins

• First-trimester scan to determine chorionicity and amnionicity 5, 6
• Anatomy scan at 18-22 weeks 5, 6
• Serial growth scans every 3-4 weeks starting from the anatomy scan 5, 6

Monochorionic Twins

• Surveillance begins at 16 weeks with weekly to biweekly monitoring for twin-twin transfusion syndrome (TTTS) 5
• Fetal biometry every 2-3 weeks 5, 6
• Weekly assessment of amniotic fluid volumes and bladder visualization to screen for TTTS 5
• Umbilical artery Doppler evaluation should be incorporated 5
• More intensive surveillance is required due to higher complication rates including TTTS, twin anemia-polycythemia sequence, and selective growth restriction 5

Preeclampsia

At first diagnosis of preeclampsia, perform fetal biometry, amniotic fluid volume assessment, and umbilical artery Doppler 5:

• Serial evaluation every 2 weeks from 24-26 weeks until birth if fetal growth restriction is present 5
• Weekly or more frequent scans if umbilical artery Doppler shows increased resistance (pulsatility index >95th percentile) or absent/reversed end-diastolic flow 5

Maternal Obesity (BMI ≥35 kg/m²)

• Early anatomy assessment at 14-16 weeks may be considered to reduce acoustic window problems 5
• Standard anatomy scan at 20-22 weeks (delayed 2 weeks from standard timing) 5, 1
• Growth scan at 28-32 weeks to aid detection of late-onset fetal growth restriction when clinical assessment is limited 5

Common Pitfalls to Avoid

• Do not perform unnecessary follow-up scans for isolated soft markers (echogenic intracardiac focus, choroid plexus cysts) when aneuploidy screening is negative 1
• Do not delay the anatomy scan beyond 22 weeks as this limits clinical options if abnormalities are detected 2
• Do not use last menstrual period dating when ultrasound dating is available, as ultrasound is more accurate and improves screening sensitivity 2, 3
• Do not apply singleton surveillance protocols to monochorionic twins, as they require substantially more intensive monitoring 5
• Do not rely on umbilical artery Doppler as a screening tool in uncomplicated pregnancies; it is useful only once complications are identified 5, 6

Evidence Quality Considerations

The recommendations for standard singleton pregnancy ultrasound schedule are based on moderate-quality evidence from multiple large randomized trials 3, 4. The evidence for multiple gestation surveillance comes from high-quality guidelines from maternal-fetal medicine societies 5. Long-term follow-up studies show no detrimental effects of ultrasound on children's physical or cognitive development 3, 4.