# 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).

![Transvaginal ultrasound of a 7-week embryo demonstrating crown-rump length measurement, yolk sac, and surrounding gestational sac](first-trimester-embryo-crl.png)

## 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.

![Standard axial views for fetal biometry showing BPD, HC, AC, and femur length measurement planes](fetal-biometry-standard-views.png)

## 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.

![Fetal ultrasound demonstrating the lemon sign and banana sign associated with open spina bifida at 20 weeks gestation](lemon-banana-signs-spina-bifida.png)

## 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

1. 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.
2. ISUOG Practice Guidelines: Performance of First-Trimester Fetal Ultrasound Scan. *Ultrasound Obstet Gynecol*. 2013;41(1):102-113.
3. 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.
4. Norton ME, Scoutt LM, Feldstein VA. *Callen's Ultrasonography in Obstetrics and Gynecology*. 6th ed. Elsevier; 2017.