What type of ultrasound is recommended at 13 weeks gestation?

Ultrasound Recommendations at 13 Weeks Gestation

At 13 weeks gestation, a transabdominal ultrasound with nuchal translucency (NT) measurement is the recommended standard examination, supplemented by transvaginal ultrasound when necessary for better visualization of fetal anatomy. 1

Primary Components of 13-Week Ultrasound

• A transabdominal ultrasound should be performed to assess fetal viability, number of fetuses, biometry, and nuchal translucency measurement 1
• Transvaginal ultrasound should be added when transabdominal imaging is suboptimal or incomplete, as it significantly improves visualization of cranium, spine, stomach, kidneys, bladder, and limbs 2
• Nuchal translucency measurement is essential at this gestational age as it serves as a screening tool for chromosomal abnormalities 1

Detailed Anatomical Assessment

• Some practices perform a detailed first trimester obstetric ultrasound between 12 weeks 0 days and 13 weeks 6 days 1
• First trimester detection of structural abnormalities is possible in approximately 27.3% of cases with anomalies 1
• Common anomalies detectable at 13 weeks include defects of the cranial vault, midline brain, and abdominal wall 1
• Major anomalies with high detection rates at this stage include acrania, holoprosencephaly, encephalocele, exomphalos, gastroschisis, and body-stalk anomaly 3

Limitations of 13-Week Ultrasound

• Cardiac and renal structures may not be adequately visualized at 13 weeks (heart not properly visualized in 42% of cases and kidneys in 27% of cases) 2
• A complete fetal anatomical survey can be achieved by transabdominal ultrasound in approximately 64% of cases versus 82% when transvaginal ultrasound is added 2
• Patient body mass index significantly affects the ability to achieve a complete anatomical survey 2

Special Considerations

• For multiple gestations, the 13-week scan should include assessment of chorionicity, amnionicity, and intertwin membrane 1
• In multiple gestations, discrepant crown-rump length (CRL) measurements may be an early marker for twin-to-twin transfusion syndrome 1
• Fetal echocardiography can be performed between 12 weeks 0 days and 13 weeks 6 days in high-risk cases (e.g., previous child with congenital anomaly, maternal diabetes, IVF pregnancy, multiple gestations) 1

Follow-up Recommendations

• Despite the value of early scanning, a second-trimester anomaly scan (18-22 weeks) remains essential for complete fetal anatomical assessment 3, 2
• The detection rate of structural fetal anomalies increases from approximately 22% at first-trimester scan to 69% when combined with second-trimester scan 4

Clinical Pitfalls to Avoid

• Doppler studies (middle cerebral artery, umbilical artery) are not recommended at 13 weeks as they have no established role at this gestational age 1
• Biophysical profile has no role in the first trimester and should not be performed 1
• Relying solely on the 13-week scan without follow-up second-trimester assessment would miss a significant number of anomalies that become apparent later 3, 2