First Trimester Ultrasound Measurements: CRL, NT, and Gestational Age Reference
Complete first trimester ultrasound measurement reference — crown-rump length, nuchal translucency, gestational age dating, and what findings to document and flag in early pregnancy scanning.
First trimester ultrasound carries some of the highest clinical stakes in obstetric sonography. You're confirming viability, establishing accurate dating, ruling out ectopic pregnancy, and screening for chromosomal abnormalities — all in one scan. The measurements you record directly influence decisions about nuchal translucency screening, Down syndrome risk, and delivery planning.
This guide covers the key first trimester ultrasound measurements you need to know, with reference values and the clinical context to apply them correctly.
Gestational Age: LMP vs. Ultrasound
Gestational age (GA) is counted from the first day of the last menstrual period (LMP) — not from conception. At 6 weeks LMP, the embryo is only about 4 weeks post-conception.
Ultrasound dating is most accurate in the first trimester (CRL) and becomes progressively less precise as pregnancy advances:
| Trimester | Measurement | Accuracy |
|---|---|---|
| 6–13 weeks | Crown-rump length (CRL) | ± 5–7 days |
| 14–20 weeks | BPD, HC, AC, FL | ± 7–10 days |
| After 20 weeks | Composite biometry | ± 2–3 weeks |
| After 28 weeks | Composite biometry | ± 3–4 weeks |
Crown-Rump Length (CRL)
The CRL is the gold standard measurement for first trimester dating and is obtained between approximately 6–13 weeks.
Measurement Technique
- Plane: Neutral position — neither flexed nor extended
- Calipers: From the top of the head (crown) to the bottom of the rump, not including the yolk sac or legs
- Position: Midsagittal plane of the embryo/fetus
- Best approach: Measure at maximum length in neutral position; take 3 measurements, use the average
CRL Normal Values and Gestational Age Correlation
| CRL (mm) | Approximate Gestational Age |
|---|---|
| 2 mm | 5 weeks 5 days |
| 5 mm | 6 weeks 2 days |
| 10 mm | 7 weeks 0 days |
| 16 mm | 7 weeks 5 days |
| 25 mm | 8 weeks 6 days |
| 35 mm | 9 weeks 5 days |
| 45 mm | 10 weeks 4 days |
| 60 mm | 11 weeks 3 days |
| 75 mm | 12 weeks 3 days |
| 84 mm | 13 weeks 0 days |
Dating caveat: When CRL-based dating differs from LMP by > 7 days (first trimester), the EDD should be adjusted to the ultrasound date per ACOG guidelines.
Gestational Sac (GS)
The gestational sac is the first structure identified on ultrasound in pregnancy, typically visible at 4.5–5 weeks.
Mean Sac Diameter (MSD)
Measured as the average of three orthogonal diameters: MSD = (length + width + height) / 3
| MSD | Expected Contents |
|---|---|
| 10 – 20 mm | Yolk sac should be visible |
| ≥ 25 mm | Embryo with cardiac activity should be visible |
Diagnostic threshold for failed pregnancy: MSD ≥ 25 mm with no embryo = likely failed pregnancy (anembryonic gestation / blighted ovum). Confirm with follow-up before clinical action.
Yolk Sac
| Parameter | Normal Value |
|---|---|
| Size | 3 – 6 mm |
| Appearance | Round, echogenic ring, anechoic center |
| Timing | Visible from ~5.5 weeks until ~12 weeks |
Abnormal yolk sac findings: Size > 6 mm, irregular shape, echogenic/calcified, or absent when expected — all associated with higher risk of pregnancy loss.
Cardiac Activity
Cardiac activity (fetal heartbeat) should be visible by transvaginal ultrasound (TVUS) when:
- Embryo is ≥ 7 mm (CRL) — if no heartbeat, findings are concerning for failed pregnancy
- After 6 weeks 3 days LMP in most normal pregnancies
Normal Fetal Heart Rate by Gestational Age
| Gestational Age | Normal FHR |
|---|---|
| 6 weeks | 90 – 110 bpm |
| 7 weeks | 120 – 160 bpm |
| 8–10 weeks | 150 – 175 bpm |
| 10–14 weeks | 140 – 160 bpm |
Bradycardia in first trimester: FHR < 100 bpm at ≥ 7 weeks is a significant risk factor for pregnancy loss.
Nuchal Translucency (NT)
Nuchal translucency is the key aneuploidy screening measurement of the first trimester, performed between 11 weeks 0 days and 13 weeks 6 days (CRL 45–84 mm).
Measurement Technique (Strict Protocol)
NT measurement accuracy is highly technique-dependent. Key requirements:
- CRL must be 45–84 mm (outside this range, NT is not performed)
- Neutral head position (not flexed or hyperextended)
- Midsagittal view of the fetal profile
- Magnify until the fetal head and thorax fill the image
- Calipers inside the two echogenic lines of the nuchal fold (not on the membrane)
- Measure at the widest point
- Take three measurements, use the largest
NT Reference Values
| CRL | 95th Percentile NT |
|---|---|
| 45 mm | 2.0 mm |
| 55 mm | 2.1 mm |
| 65 mm | 2.2 mm |
| 75 mm | 2.3 mm |
| 84 mm | 2.5 mm |
NT ≥ 3.0 mm is widely used as an elevated threshold, though risk should be reported as a number (e.g., combined first trimester screen risk of 1:250) rather than a pass/fail cutoff. An elevated NT with a normal karyotype may still indicate cardiac or structural abnormalities requiring fetal echo.
Additional First Trimester Findings to Document
Nasal Bone
- Absent nasal bone at 11–14 weeks is associated with trisomy 21
- Present = echogenic linear structure at the bridge of the nose
- Absent or hypoplastic = technically challenging, requires experience and proper angle
Ductus Venosus Waveform
In specialized centers, ductus venosus (DV) Doppler is used for aneuploidy risk refinement:
- Normal: forward flow during atrial contraction (positive a-wave)
- Abnormal: absent or reversed a-wave = elevated risk for trisomy 21/18/13
Fetal Anatomy (Limited, 12–14 weeks)
- Four-chamber heart view (basic)
- Abdominal wall integrity (rule out omphalocele)
- Spine (limited assessment)
- Limbs (gross assessment)
Key Images for Your Worksheet
- Gestational sac with MSD measurement (three planes)
- Yolk sac — size and appearance
- Embryo/fetus — CRL measurement (neutral position, 3 measurements)
- Cardiac activity — M-mode or cine clip with heart rate
- Nuchal translucency (if 11–14 weeks) — with measurements
- Uterus and adnexa — rule out ectopic, fibroids, ovarian cysts
- Cervical length (if clinically indicated)
Quick Reference Summary
| Parameter | Normal Value / Threshold |
|---|---|
| Gestational sac visible | ≥ 4.5 weeks LMP |
| Yolk sac visible | ≥ 5.5 weeks LMP |
| Cardiac activity visible | CRL ≥ 7 mm; ≥ 6.3 weeks |
| Normal FHR at 7–10 wks | 120 – 175 bpm |
| CRL dating accuracy | ± 5–7 days |
| NT window | CRL 45–84 mm (11w0d–13w6d) |
| NT elevated threshold | ≥ 3.0 mm (risk-based counseling) |
| MSD threshold (no embryo) | ≥ 25 mm = concerning |
Use SonoBuddy for OB Calculations
SonoBuddy's OB calculators include:
- Gestational Age & EDD from LMP or CRL
- Expected dating from CRL with GA output
Access under Calculators → Gestational Age.
References: ACOG Practice Bulletin No. 175 (Ultrasound in Pregnancy). ISUOG Practice Guidelines (first trimester scan). Hadlock FP et al. (dating tables). Nicolaides KH (NT screening). Society of Maternal-Fetal Medicine.
SonoBuddy is a reference tool, not a diagnostic authority. Clinical decisions must involve the ordering provider and interpreting physician.
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