Residency · Residency · Diagnostic Radiology
Obstetric Ultrasound: First Trimester and Fetal Anatomy Survey
Introduction
Obstetric ultrasound is the cornerstone of prenatal imaging. The first trimester ultrasound (up to 13 weeks 6 days) establishes viability, number of gestations, and accurate dating. The second trimester fetal anatomy survey (18-22 weeks) provides a systematic evaluation of fetal structures to identify congenital anomalies. Every radiology resident must be proficient in the standard views and measurements required by the American Institute of Ultrasound in Medicine (AIUM) guidelines.
First Trimester Ultrasound
Indications
The first trimester ultrasound confirms intrauterine pregnancy (IUP) and viability, determines gestational age, evaluates for ectopic pregnancy, assesses the number of gestations and chorionicity/amnionicity, and evaluates nuchal translucency (NT) screening between 11 and 14 weeks.
Key Landmarks and Timeline
The gestational sac is visible at approximately 4.5-5 weeks (mean sac diameter 2-3 mm), located eccentrically within the decidua (the intradecidual sign). The yolk sac is the first structure seen within the gestational sac, typically by 5.5 weeks, and should measure less than 6 mm. The embryo with cardiac activity is detectable at approximately 6 weeks when the crown-rump length (CRL) reaches 2-4 mm. The amnion is visible as a thin membrane surrounding the embryo by 7 weeks.
Diagnostic Criteria for Early Pregnancy Failure
| Criterion | Finding | Interpretation |
|---|---|---|
| CRL >=7 mm | No cardiac activity | Definitive pregnancy failure |
| MSD >=25 mm | No embryo | Definitive pregnancy failure |
| GS without yolk sac | No embryo with heartbeat >=2 weeks later | Definitive pregnancy failure |
| GS with yolk sac | No embryo with heartbeat >=11 days later | Definitive pregnancy failure |
Definitive criteria include a CRL of 7 mm or greater with no cardiac activity, or a mean sac diameter of 25 mm or greater with no embryo. Additional criteria include absence of an embryo with heartbeat 2 or more weeks after a scan showing a gestational sac without yolk sac, or absence of an embryo with heartbeat 11 or more days after a scan showing a gestational sac with yolk sac.
Dating
CRL is the most accurate dating parameter in the first trimester, with accuracy of plus or minus 5-7 days. If CRL dating differs from the LMP by more than 7 days, the ultrasound date should be used as the estimated due date (EDD).
Multiple Gestations: Chorionicity and Amnionicity
Chorionicity and amnionicity must be determined in the first trimester when most reliably assessed. Dichorionic-diamniotic (DCDA) twins show the twin peak sign (lambda sign), a triangular projection of placental tissue extending into the base of the inter-twin membrane. Monochorionic-diamniotic (MCDA) twins show the T-sign, a thin inter-twin membrane inserting perpendicular to the placenta without a wedge of tissue. Monochorionic-monoamniotic (MCMA) twins have no inter-twin membrane and carry the highest risk for cord entanglement. MCDA twins are at risk for twin-to-twin transfusion syndrome (TTTS).
Nuchal Translucency (NT) Screening
NT is measured between 11 weeks 0 days and 13 weeks 6 days (CRL 45-84 mm) in the midsagittal plane, with calipers placed on the inner borders of the nuchal lucency. Normal is less than 3.0 mm. Increased NT is associated with trisomy 21, trisomy 18, trisomy 13, Turner syndrome, and congenital heart defects.
Second Trimester Fetal Anatomy Survey
Standard Views (AIUM Guidelines)
Head and Brain
Biparietal diameter (BPD) and head circumference (HC) are measured at the level of the thalami and cavum septum pellucidum. Lateral ventricles are measured at the atrium at the level of the choroid plexus glomus, with normal measuring less than 10 mm (10 mm or greater constitutes ventriculomegaly). The cerebellum transverse diameter approximates gestational age in weeks up to 22 weeks. The cisterna magna normally measures 2-10 mm. The nuchal fold is measured at 15-22 weeks, with 6 mm or greater associated with trisomy 21.
Face
The lips and nose are evaluated in the coronal view for cleft lip assessment.
Spine
Longitudinal and transverse views assess vertebral body alignment and skin line integrity.
Chest and Heart
The four-chamber view of the heart is obtained along with left and right ventricular outflow tracts (LVOT, RVOT). Lung echogenicity is assessed.
Abdomen
The abdominal circumference (AC) is measured at the level of the stomach, umbilical vein, and spine. The stomach (fluid-filled, left-sided), kidneys bilaterally, and bladder are evaluated. The cord insertion site and three-vessel cord (two arteries, one vein) are documented.
Extremities
Presence and morphology of long bones are assessed. Femur length (FL) is used in biometric dating.
Other
Placental location and relationship to the internal cervical os are documented. Amniotic fluid volume is assessed using the deepest vertical pocket or AFI. Cervical length is measured, with 25 mm or greater considered normal.
Common Abnormalities Detected on Anatomy Survey
Anencephaly presents as an absent calvarium and cerebral hemispheres above the orbits. Spina bifida is associated with the lemon sign (frontal bone scalloping) and banana sign (cerebellar compression). Congenital diaphragmatic hernia shows stomach or bowel in the thorax with mediastinal shift. Gastroschisis is a right paraumbilical abdominal wall defect with free-floating bowel loops. Omphalocele is a midline defect with membrane-covered herniated contents including liver.
Key Clinical Pearls
Always determine chorionicity in the first trimester for twin pregnancies because it dictates surveillance and management. A short CRL with no heartbeat does not always mean pregnancy failure -- reimaging in 7-14 days before diagnosing miscarriage is appropriate. The four-chamber heart view alone detects only approximately 50% of congenital heart defects; outflow tract views significantly increase sensitivity. Placenta previa diagnosed before 20 weeks often resolves; follow-up at 32-36 weeks is recommended. Always report the number of umbilical cord vessels because a two-vessel cord (single umbilical artery) is associated with renal and cardiac anomalies.
References
- American Institute of Ultrasound in Medicine. AIUM Practice Parameter for the Performance of Standard Diagnostic Obstetric Ultrasound Examinations. J Ultrasound Med. 2018;37(11):E13-E24.
- ISUOG Practice Guidelines: Performance of First-Trimester Fetal Ultrasound Scan. Ultrasound Obstet Gynecol. 2013;41(1):102-113.
- Doubilet PM, Benson CB, Bourne T, et al. Diagnostic Criteria for Nonviable Pregnancy Early in the First Trimester. N Engl J Med. 2013;369(15):1443-1451.
- Norton ME, Scoutt LM, Feldstein VA. Callen's Ultrasonography in Obstetrics and Gynecology. 6th ed. Elsevier; 2017.
For the intern
At 3am, focus on whether the pregnancy is intrauterine, viable, and if there's an emergency. Confirm intrauterine pregnancy to exclude ectopic. Check for cardiac activity if crown-rump length is 7 mm or greater. Document number of gestations and measure crown-rump length for accurate dating. Measure nuchal translucency only between 11-13+6 weeks when the patient is positioned and the image quality is optimal. For anatomy surveys, systematic evaluation can wait until morning with your attending unless there's clinical concern for a specific anomaly affecting immediate management. Most second-trimester anatomy findings require multidisciplinary discussion, not middle-of-the-night calls.
Common pitfalls
- Calling pregnancy failure too early: A gestational sac must be 25 mm or greater with no embryo, or crown-rump length 7 mm or greater with no cardiac activity for definitive diagnosis. When uncertain, schedule follow-up imaging in 7-14 days rather than diagnosing miscarriage prematurely.
- Missing chorionicity determination: Assess the twin peak sign versus T-sign in the first trimester. Waiting until the second trimester makes this assessment unreliable, and chorionicity dramatically changes management and surveillance protocols.
- Inadequate four-chamber heart view: The four-chamber view alone misses half of congenital heart defects. Always attempt left and right ventricular outflow tract views during the anatomy survey.
- Reporting early placenta previa as definitive: Placental position before 20 weeks frequently changes as the lower uterine segment develops. Document the finding but reassure that follow-up imaging at 32-36 weeks is standard.