What is involved in a second trimester ultrasound?

Second Trimester Ultrasound: A Pictorial Guide

Optimal Timing and Approach

The second trimester anatomic ultrasound should be performed between 20-22 weeks gestation, as this timing provides the highest completion rates (88-90%) and minimizes the need for repeat scanning compared to earlier gestational ages. 1

• Performing the scan at 18 weeks results in significantly lower completion rates (76%) due to higher incidence of non-cephalic presentation (46% vs 36%) and difficulty visualizing thorax, heart, spine, and skeletal structures. 1
• The examination is performed transabdominally using high-resolution real-time ultrasound with systematic evaluation of fetal anatomy. 2
• Transvaginal ultrasound can supplement transabdominal scanning when fetal parts are close to the cervix or in obese women between 12-16 weeks to improve anatomic visualization. 2

Standard Anatomic Survey Components

Head and Brain

• Measure biparietal diameter (BPD) and head circumference in the standard axial plane at the level of the thalami and cavum septi pellucidi. 3
• Visualize the lateral ventricles, choroid plexus, midline falx, cavum septi pellucidi, and posterior fossa including cerebellum and cisterna magna. 4
• Transverse cerebellar diameter (TCD) is particularly useful as it is relatively spared in growth restriction. 3

Face and Neck

• Document the profile view to assess for facial clefts, micrognathia, and nasal bone presence. 2
• Evaluate for nuchal fold thickness, which should be ≤6 mm; thickness >6 mm has 34% sensitivity for Down syndrome with 1.5% false positive rate. 3
• Screen for cystic hygroma and other neck masses. 2

Spine

• Examine the spine in both sagittal and axial planes from cervical to sacral regions to exclude neural tube defects. 4
• Verify intact skin covering and normal alignment of vertebral bodies. 4

Thorax

• Perform four-chamber heart view to assess cardiac size, position, axis, and chamber symmetry. 4
• Evaluate for diaphragmatic hernias and congenital pulmonary airway malformations. 2
• Document normal lung echogenicity and exclude pleural effusions. 4

Abdomen

• Visualize stomach bubble in left upper quadrant and assess abdominal wall integrity. 4
• Screen for gastroschisis and omphalocele by confirming intact abdominal wall and normal cord insertion. 2
• Measure abdominal circumference at the level of stomach and umbilical vein. 3
• Evaluate kidneys bilaterally and bladder; measure renal pelvis diameter (pyelectasis if >4mm). 2

Extremities

• Measure femur length and humerus length; short long bones (below expected for gestational age) have 30% sensitivity for aneuploidy with <5% false positive rate. 3
• Document presence of all four limbs with normal bone mineralization. 2
• Evaluate hands and feet when visible. 4

Placenta and Amniotic Fluid

• Document placental location; note that low-lying placenta is more commonly identified at 18 weeks and often resolves by term. 1
• Assess amniotic fluid volume qualitatively or quantitatively. 4

Soft Markers and Their Significance

When soft markers are identified, their clinical significance depends heavily on whether the patient has already undergone genetic screening (first trimester screening, cell-free DNA, or invasive testing). 2

Common Soft Markers

• Echogenic intracardiac focus, choroid plexus cysts, renal pyelectasis, short humerus/femur, nuchal thickening (≥6mm), echogenic bowel, and short/absent nasal bone are associated with aneuploidy risk. 2
• For isolated echogenic intracardiac focus or choroid plexus cyst alone, detailed anatomic scan is optional if the finding is truly isolated. 2
• For other soft markers (renal pyelectasis, short bones, nuchal thickening, echogenic bowel, absent nasal bone), a detailed scan is usually indicated to ensure the finding is isolated. 2

Important Context

• If the patient has had normal karyotype analysis (CVS/amniocentesis) or cell-free DNA testing, soft markers lose their relevance for aneuploidy risk and should not alter management. 2
• The introduction of first trimester screening and NIPT has reduced the clinical utility of soft markers, with greater impact on false positive rates than detection rates. 2

When to Perform Detailed/Targeted Ultrasound

A detailed fetal anatomic examination is indicated when: 2

• Major anomalies are suspected or identified on routine screening
• The patient has high-risk factors (maternal medical conditions, family history, abnormal genetic screening)
• Soft markers are identified and require confirmation of isolated finding
• The routine anatomy scan is incomplete due to technical factors

Complementary Imaging

Fetal Echocardiography

• Indicated based on parental risk factors (maternal diabetes, genetic syndromes, teratogen exposure) or abnormal cardiac screening on anatomy scan. 2

Fetal MRI

• Generally indicated when ultrasound findings are incomplete or require additional detail, ideally performed at or after 22 weeks gestation. 2
• Particularly helpful for central nervous system anomalies, neck masses requiring airway planning, and complex thoracic/abdominal pathology. 2
• MRI without contrast only; contrast is not indicated for fetal evaluation. 2

Safety Considerations

• Long-term follow-up studies show no detrimental effects on cognitive or physical development in children exposed to ultrasound in utero. 2, 5
• Highest energy output occurs with Doppler ultrasound; familiarity with safety protocols is recommended. 2
• Doppler imaging should be used judiciously to assess vessels and organs when clinically indicated. 2

Detection Rates

• Second trimester ultrasound detects 55% of major malformations before 24 weeks in unselected populations. 2
• Detection improves to 75-90% for open neural tube defects and 95% for anencephaly when combined with maternal serum alpha-fetoprotein screening. 2
• Routine second trimester ultrasound increases detection of major fetal abnormalities before 24 weeks (RR 3.45) and termination for major anomaly (RR 2.36). 5

Common Pitfalls to Avoid

• Do not perform anatomy scan at 18 weeks routinely; wait until 20-22 weeks to maximize completion rates and minimize repeat scans. 1
• Ensure accurate gestational age dating before interpreting measurements; ultrasound dating in first trimester is most accurate (±5-7 days). 6, 7
• Do not over-interpret soft markers in patients who have had normal genetic screening or invasive testing. 2
• Recognize limitations including maternal obesity, fetal position, and oligohydramnios that may necessitate repeat scanning. 2