Residency · Residency · Obstetrics Gynecology
Second-Trimester Anatomy Survey and Soft Markers
Introduction
The second-trimester anatomy survey, typically performed between 18 and 22 weeks of gestation, is the most comprehensive prenatal ultrasound examination. It evaluates fetal anatomy for structural anomalies, assesses placental location and amniotic fluid volume, and provides biometric measurements for growth assessment. Soft markers -- minor sonographic findings that are often variants of normal -- may modify aneuploidy risk and guide further evaluation.
Standard Anatomic Survey Components
Head and Brain
Biparietal diameter and head circumference are measured at the level of the thalami and cavum septum pellucidum in the axial plane. The atrium of the lateral ventricle is measured at the level of the choroid plexus, with normal being less than 10 mm; ventriculomegaly is defined as 10-15 mm (mild) or greater than 15 mm (severe). In the posterior fossa, the cerebellar diameter in millimeters roughly equals gestational age in weeks, and the cisterna magna normally measures 2-10 mm. The cavum septum pellucidum must be visualized; its absence suggests agenesis of the corpus callosum, septo-optic dysplasia, or holoprosencephaly. The nuchal fold, measured in the transcerebellar plane at 15-22 weeks, is associated with trisomy 21 when 6 mm or greater.
Face
Coronal views assess for cleft lip, and 3D ultrasound can aid evaluation of the secondary palate. Orbits and nasal bone are assessed for hypotelorism, hypertelorism, and absent nasal bone. The mid-sagittal profile evaluates for micrognathia and frontal bossing.
Spine
Longitudinal and axial views of the entire spine assess vertebral alignment, skin covering, and neural arch closure. Open spina bifida is associated with intracranial signs: the lemon sign (frontal bone scalloping) and banana sign (cerebellar compression), which are more reliable than direct spine visualization.
Heart
Systematic cardiac evaluation requires the four-chamber view (chamber size, septal integrity, AV valve function, cardiac axis at 45 degrees leftward), left ventricular outflow tract (aorta from left ventricle), right ventricular outflow tract (pulmonary artery from right ventricle, crossing over the aorta), and three-vessel-trachea view (relative size and position of pulmonary artery, aorta, and SVC). Fetal echocardiography is indicated when abnormalities are suspected, NT was elevated, or risk factors exist.
<image>Composite ultrasound illustration showing the standard cardiac views in the second-trimester anatomy survey: four-chamber view, left ventricular outflow tract, right ventricular outflow tract, and three-vessel-trachea view, with key structures labeled</image>
Thorax and Abdomen
Lungs should appear homogeneously echogenic. The stomach should be fluid-filled in the left upper quadrant; an absent stomach bubble raises concern for esophageal atresia. Kidneys are visualized bilaterally with renal pelvis AP diameter greater than 4 mm suggesting pyelectasis. Bilateral renal agenesis presents with anhydramnios and non-visualization of kidneys or bladder. The abdominal wall is assessed at the cord insertion for omphalocele (midline, membrane-covered) and gastroschisis (right paraumbilical, no membrane). A single umbilical artery (present in 1% of pregnancies) is associated with renal and cardiac anomalies.
Extremities
Femur and humerus are measured and compared to expected values. Short measurements may indicate skeletal dysplasia or aneuploidy. Hands and feet are assessed for polydactyly, syndactyly, and clubfoot.
Placenta and Amniotic Fluid
Placental location is documented; previa is diagnosed when the placenta covers or is within 2 cm of the internal os (most low-lying placentas at 18-22 weeks resolve by term). Amniotic fluid is assessed by AFI or single deepest pocket; oligohydramnios (AFI less than 5 cm or SDP less than 2 cm) and polyhydramnios (AFI greater than 24 cm or SDP greater than 8 cm) warrant further evaluation.
<image>Ultrasound illustration showing a standard axial view of the fetal brain at the level of the lateral ventricles, with the measurement of the atrial width, choroid plexus, cavum septum pellucidum, and thalami labeled</image>
Soft Markers for Aneuploidy
Definition and Clinical Significance
Soft markers are minor sonographic findings that occur more frequently in aneuploid fetuses but are often transient or normal variants. Their significance has diminished with cell-free DNA screening (cfDNA), which provides superior sensitivity for common trisomies. An isolated soft marker in a patient with low-risk cfDNA results generally does not warrant invasive testing.
Individual Soft Markers
Echogenic intracardiac focus (EIF), seen in 3-5% of normal fetuses, has a positive likelihood ratio of approximately 1.8 for trisomy 21 when isolated. Choroid plexus cysts (1-2% of fetuses) are associated with trisomy 18 when multiple or bilateral but are benign when isolated with normal anatomy. Echogenic bowel (equal to or greater than adjacent bone) warrants workup for aneuploidy, cystic fibrosis, intrauterine infection, and placental insufficiency. Mild pyelectasis (4-7 mm renal pelvis) has a likelihood ratio of approximately 1.5. Thickened nuchal fold (6 mm or greater) is the strongest individual soft marker for trisomy 21 with a likelihood ratio of 11-17. Absent or hypoplastic nasal bone is present in approximately 50-60% of trisomy 21 fetuses.
| Soft Marker | Prevalence in Normal Fetuses | LR+ for Trisomy 21 | Primary Association | Isolated Finding Management |
|---|---|---|---|---|
| Echogenic intracardiac focus | 3-5% | ~1.8 | Trisomy 21 | Reassurance if low-risk screening |
| Choroid plexus cyst | 1-2% | ~1.0 | Trisomy 18 | Benign if isolated with normal anatomy |
| Echogenic bowel | 0.5-1% | 6.7 | Trisomy 21, CF, infection, placental insufficiency | Further workup recommended |
| Mild pyelectasis (4-7 mm) | 2-3% | ~1.5 | Trisomy 21 | Follow-up in third trimester |
| Thickened nuchal fold (≥6 mm) | 0.5% | 11-17 | Trisomy 21 | Genetic counseling, consider diagnostic testing |
| Absent/hypoplastic nasal bone | 1-3% | 6.6-50 | Trisomy 21 | Genetic counseling, consider diagnostic testing |
| Short humerus/femur | 3-5% | 2.5-7.5 | Trisomy 21, skeletal dysplasia | Consider in context of other findings |
Integrated Approach
Multiple soft markers have multiplicative likelihood ratios, substantially increasing post-test probability. A structural anomaly (such as AV canal defect or duodenal atresia) is far more significant than soft markers and warrants genetic counseling and diagnostic testing.
<image>Ultrasound images showing common soft markers: echogenic intracardiac focus in the left ventricle, choroid plexus cyst in the lateral ventricle, and echogenic bowel in the fetal abdomen, each with calipers and labels</image>
Clinical Pearls
The anatomy survey is optimally performed at 18-22 weeks; earlier scans may miss anomalies not yet apparent, while later scans face limitations from fetal size and ossification. Thickened nuchal fold (6 mm or greater) is the most predictive individual soft marker for trisomy 21. Isolated choroid plexus cysts with normal anatomy and reassuring aneuploidy screening are benign and do not require invasive testing. Always evaluate cardiac outflow tracts in addition to the four-chamber view -- the four-chamber view alone detects only approximately 50% of cardiac anomalies. Placenta previa at the anatomy survey should be followed with a third-trimester ultrasound, as the majority resolve with lower uterine segment growth.
References
- American Institute of Ultrasound in Medicine. AIUM-ACR-ACOG-SMFM-SRU Practice Parameter for the Performance of Standard Diagnostic Obstetric Ultrasound Examinations. J Ultrasound Med. 2018;37(11):E13-E24.
- Norton ME, Biggio JR, Kuller JA, Blackwell SC. The role of ultrasound in women who undergo cell-free DNA screening. Am J Obstet Gynecol. 2017;216(3):B2-B7.
- Nyberg DA, Souter VL, El-Bastawissi A, et al. Isolated sonographic markers for detection of fetal Down syndrome in the second trimester of pregnancy. J Ultrasound Med. 2001;20(10):1053-1063.
- International Society of Ultrasound in Obstetrics and Gynecology. ISUOG Practice Guidelines: ultrasound assessment of fetal biometry and growth. Ultrasound Obstet Gynecol. 2019;53(6):715-723.
