SonographyAcademy

Curriculum · Clinical Ultrasound: Multisystem Diagnostic & Point-of-Care Sonography

Obstetric & First-Trimester Ultrasound: Dating, Biometry & Safety

First-trimester landmarks (GS/YS/CRL) and dating accuracy, biometry (BPD, HC, AC, FL) and estimated fetal weight, fetal cardiac screening planes, and OB-specific ALARA/TI/MI safety.

~40 min · level: advanced · POCUSOB/GYN draft — pending clinical review

Learning objectives

  • Identify first-trimester landmarks (gestational sac, yolk sac, CRL) and state dating accuracy by trimester.
  • Name the standard biometric measurements and compute gestational age from CRL and estimated fetal weight (Hadlock).
  • List the standard fetal cardiac screening views and the role of a STIC volume.
  • Apply OB-specific ALARA — thermal/mechanical index limits and the caution against spectral Doppler over the early embryo.

Obstetric ultrasound establishes viability, number, location, dating, anatomy, and growth. Transvaginal (endocavitary, high-frequency) imaging dominates the first trimester; transabdominal (curvilinear) dominates later. Accurate dating is the foundation of everything downstream (growth, post-dates management).

LandmarkTypical appearanceNote

Gestational sac (GS)

~ 4.5–5 weeks

Intradecidual/double-decidual sign; measure mean sac diameter

Yolk sac (YS)

~ 5.5 weeks (GS ~ 8–10 mm)

First structure confirming intrauterine pregnancy

Embryo + cardiac activity (CRL)

~ 6 weeks

Cardiac activity expected by CRL ≥ 7 mm

Crown–rump length (CRL)

T1

Most accurate dating measurement (± 5–7 days)

First-trimester milestones (transvaginal).
GA (days)=8.052 (CRL(mm)×1.037)1/2+23.73\mathrm{GA}\,(\text{days}) = 8.052\,(\mathrm{CRL}_{(\mathrm{mm})}\times 1.037)^{1/2} + 23.73
Robinson–Fleming: gestational age from crown–rump length (one of several validated equations; software uses regional reference charts).
MeasurementPlaneMeasures

BPD (biparietal diameter)

Axial head (thalami, CSP)

Outer-to-inner skull

HC (head circumference)

Same axial plane

Around the calvarium

AC (abdominal circumference)

Axial abdomen (stomach, portal vein)

Around the abdomen — most sensitive to growth

FL (femur length)

Full femoral diaphysis

Ossified shaft only

Standard fetal biometry (second/third trimester).
log⁡10(EFW)=1.3596−0.00386(AC⋅FL)+0.0064 HC+0.00061(BPD⋅AC)+0.0424 AC+0.174 FL\log_{10}(\mathrm{EFW}) = 1.3596 - 0.00386(\mathrm{AC}\cdot\mathrm{FL}) + 0.0064\,\mathrm{HC} + 0.00061(\mathrm{BPD}\cdot\mathrm{AC}) + 0.0424\,\mathrm{AC} + 0.174\,\mathrm{FL}
A Hadlock estimated-fetal-weight model (EFW in g; measurements in cm). Several Hadlock formulas exist; modern machines compute EFW and percentile automatically.

Fetal cardiac screening is the highest-yield anomaly screen. The clip below is a real 4-D STIC (Spatio-Temporal Image Correlation) volume of the fetal heart: a single gated volume is acquired, then navigated to display the standard planes.

4D STICFetal heart — 4D STIC volume, systematic plane navigation
Reveal findings
  • The screening sequence: (1) four-chamber view, (2) LV outflow tract, (3) RV outflow tract, (4) three-vessel & three-vessel-trachea views.
  • The four-chamber view is the cornerstone — assesses chamber symmetry, the crux, AV valves, and rate/rhythm.
  • STIC reconstructs a beating-heart volume from a slow sweep, allowing offline multiplanar review without prolonged live insonation of the fetus.

Which single view is the cornerstone of the basic fetal cardiac screening examination?

Real clinical clip · Jantarasaengaram S, Vairojanavong K · CC BY 2.0 · Eleven fetal echocardiographic planes using 4-D ultrasound with STIC — Cardiovasc Ultrasound (BMC), via Wikimedia Commons
Fetal heart — 4-D STIC volume with systematic plane navigation. From one volume the operator can reconstruct the screening planes (four-chamber, outflow tracts, three-vessel).
STIC (Spatio-Temporal Image Correlation)

A 4-D technique that acquires a slow volume sweep and, using the detected fetal heart rate, reorders the frames into a cine of a single average cardiac cycle in 3-D — enabling multiplanar, offline navigation of the fetal heart from one acquisition.

Worked example — dating discrepancy

An 8-week scan by CRL gives an EDD. A 32-week growth scan estimates a gestational age 10 days "behind" the CRL date. Do you change the due date?

Solution.

No. The earliest accurate dating (CRL in T1, ± 5–7 d) sets the EDD and is not overridden by a third-trimester biometric estimate (accuracy only ± 21–30 d). A measurement lagging the established date in T3 raises concern for fetal growth restriction, prompting growth/Doppler surveillance — not re-dating.

Key takeaways

  • Crown-rump length in the first trimester is the single most accurate dating measurement (+/- 5-7 days), and dating accuracy degrades to +/- 7-14 days by T2 biometry and only +/- 21-30 days in T3.
  • Date early and don't re-date: once an early scan sets the EDD it is not overridden by later scans, so a third-trimester measurement lagging the established date signals fetal growth restriction (surveillance), not a reason to change the due date.
  • First-trimester landmarks appear in sequence -- gestational sac around 4.5-5 weeks, yolk sac around 5.5 weeks (the first structure confirming intrauterine pregnancy), and embryo with cardiac activity expected by CRL >= 7 mm.
  • The four-chamber view is the cornerstone of fetal cardiac screening (chamber symmetry, crux, AV valves, rate/rhythm), with outflow-tract and three-vessel views added to detect conotruncal lesions, and abdominal circumference is the biometric most sensitive to growth and the largest-weighted term in Hadlock EFW.
  • OB ALARA is strictest here: prefer B-mode/M-mode in T1, avoid routine spectral (and color) Doppler over the early embryo because spectral Doppler carries the highest acoustic intensity and thermal risk, and keep TI < 0.5-1.0 when feasible.

Check your understanding

Registry-style items with worked rationales.

1The single most accurate ultrasound measurement for dating a pregnancy is:recall

2The cornerstone view of basic fetal cardiac screening is the:recall

3In the first trimester, the preferred mode to document embryonic cardiac activity while minimizing acoustic exposure is:application

4Which biometric measurement is generally the most sensitive indicator of fetal growth abnormality?analysis

5An early CRL-based scan sets an EDD; a third-trimester scan estimates an age 10 days behind. The correct action is:analysis

Go deeper — trusted free resources

Hand-picked, verified links to authoritative open resources. Opens in a new tab.

Radiopaedia
First trimester pregnancy (ultrasound overview)

Comprehensive reference on normal first-trimester sonographic milestones including the gestational sac, yolk sac, and embryonic cardiac activity with their expected timing.

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Radiopaedia
Crown-rump length (CRL)

Explains how CRL is measured and used as the most accurate method of pregnancy dating in the first trimester.

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Radiopaedia
Thermal index (ultrasound safety)

Covers the thermal index, its bioeffects, and why color/spectral Doppler should be limited when imaging the early embryo for obstetric safety.

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ISUOG
ISUOG Practice Guidelines: performance of first-trimester fetal ultrasound scan

Authoritative society guideline detailing the technique, biometry, and safety standards for the first-trimester obstetric scan.

Open →

References

  1. Salomon LJ, Alfirevic Z, et al. ISUOG practice guidelines: performance of first-trimester and mid-trimester fetal ultrasound scans. Ultrasound Obstet Gynecol.
  2. AIUM Practice Parameters (obstetric ultrasound; as-low-as-reasonably-achievable / bioeffects statements).
  3. Hadlock FP, Harrist RB, Sharman RS, et al. Estimation of fetal weight with the use of head, body, and femur measurements. Am J Obstet Gynecol. 1985;151(3):333-337.
  4. Robinson HP, Fleming JEE. A critical evaluation of sonar crown-rump length measurements. Br J Obstet Gynaecol. 1975;82(9):702-710.