First Trimester Ultrasound Protocol: What to Scan and Document

The first trimester ultrasound is one of the most clinically important studies in obstetrics. It confirms intrauterine pregnancy, establishes gestational age, evaluates for early complications, and screens for chromosomal abnormalities. Here's a systematic approach.

Indications

• Confirm intrauterine pregnancy
• Establish gestational age (crown-rump length dating is most accurate in first trimester)
• Evaluate for threatened abortion, missed abortion, blighted ovum
• Evaluate for ectopic pregnancy
• Nuchal translucency screening (11w0d – 13w6d)
• First trimester anatomy screen (when performed)
• Evaluate subchorionic hematoma
• Evaluate pelvic pain or bleeding in early pregnancy

Transducer Choice

Transvaginal (TVS): Preferred in early first trimester (before 10–11 weeks). Superior resolution for small structures. Sees the gestational sac 1–2 weeks earlier than transabdominal.

Transabdominal (TAS): Used after 10–12 weeks when the uterus is rising out of the pelvis. Also used if TVS is declined or not appropriate.

Most early first trimester studies begin transvaginally and may finish transabdominally.

Systematic Scanning Approach

Step 1: Uterus Overview

Before looking for the pregnancy, survey the uterus:

• Is it anteverted or retroverted?
• Any fibroids? (Document size and location — can affect pregnancy)
• Endometrial appearance

Step 2: Gestational Sac

The gestational sac (GS) is the first sonographic sign of pregnancy.

Normal GS:

• Round or oval anechoic structure with echogenic rim (decidual reaction)
• Located in the uterine fundus or mid-cavity — not in the lower uterine segment or cervix
• Mean sac diameter (MSD): measure in three planes, average them
• Visible on TVS at approximately 4.5–5 weeks (when MSD ≈ 2–3 mm)

Empty sac without yolk sac: Normal until MSD ≈ 8–10 mm. If MSD > 16–20 mm without a yolk sac, this is a failed pregnancy (blighted ovum / anembryonic pregnancy).

Double decidual sac sign: Two echogenic rings around the GS — the inner decidua capsularis and outer decidua parietalis. Helps distinguish IUP from a pseudogestational sac seen in ectopic pregnancy.

Step 3: Yolk Sac

The yolk sac is the first structure visible inside the GS and confirms intrauterine pregnancy.

• Visible on TVS at approximately 5–5.5 weeks
• Appears as a round echogenic ring with an anechoic center
• Normal diameter: 3–6 mm
• Should be visible when MSD > 10–12 mm
• Large yolk sac (> 6 mm) or calcified/irregular yolk sac — associated with poor outcomes

Step 4: Embryo and Cardiac Activity

The embryo is visible as a small echogenic structure adjacent to the yolk sac.

Crown-rump length (CRL): Measure the longest axis of the embryo, excluding the yolk sac. Use calipers at the natural curvature of the embryo, not stretched or compressed.

Gestational age from CRL:

• CRL 3–84 mm corresponds to approximately 6w0d – 14w0d
• Most accurate dating method in pregnancy

Cardiac activity:

• Should be visible when CRL ≥ 7 mm (TVS)
• If CRL ≥ 7 mm and no cardiac activity — likely failed pregnancy (embryonic demise)
• Document heart rate — normal: 100–170 bpm by 8–10 weeks
• Bradycardia (< 100 bpm before 8 weeks) is a poor prognostic sign

M-mode for heart rate: Use M-mode rather than spectral Doppler for cardiac rate in first trimester (reduces acoustic output exposure). Measure at least 3 cycles.

Step 5: Uterine Adnexa

Always evaluate both adnexa in early pregnancy:

• Ovaries: size, appearance, corpus luteum (normal finding — supports the pregnancy)
• Adnexal masses: evaluate any cysts > 3 cm
• Free fluid in the cul-de-sac

Any adnexal mass in conjunction with an empty uterus or abnormal sac requires careful evaluation for ectopic pregnancy.

Step 6: Nuchal Translucency (11w0d – 13w6d, CRL 45–84 mm)

NT screening requires specific training and quality review (most sites require FMF or similar certification).

Technique:

• Neutral position of fetal head (not flexed or extended)
• Mid-sagittal view of the fetus — must see nasal bone, palate, and corpus callosum
• Zoom so the fetal head and chest fill the screen
• Measure the maximum thickness of the fluid-filled space behind the fetal neck
• Place calipers on-to-on (inner edge of the echogenic skin to inner edge of the echogenic line of the neck)
• Measure at the widest point, not including the amnion

Normal NT: < 3.0 mm (varies by CRL — use reference chart) Elevated NT: ≥ 3.0 mm — associated with chromosomal abnormalities and structural defects

Subchorionic Hematoma

A subchorionic hematoma (SCH) appears as a hypoechoic or heterogeneous collection between the chorionic membrane and the uterine wall.

Document:

• Size (three dimensions)
• Location relative to the placenta and cervix
• Appearance (hypoechoic = acute; echogenic = organizing)
• % of GS circumference involved

Large SCH covering more than 25–50% of the GS has a higher risk of adverse outcome.

Failed Pregnancy Criteria (AIUM/ACR guidelines)

Finding	Criteria
No cardiac activity	CRL ≥ 7 mm (TVS)
No embryo	MSD ≥ 25 mm (TVS) without embryo
No yolk sac	MSD ≥ 10 mm without yolk sac (concerning)
No growth	No interval growth on follow-up after ≥ 11 days

These criteria have high specificity but always use clinical correlation and consider follow-up imaging before final diagnosis.

Key Images to Capture

1. Uterus — sagittal and transverse overview
2. Gestational sac — transverse with MSD measurement (3 planes)
3. Yolk sac — close-up with measurement
4. Embryo — CRL measurement (sagittal)
5. Cardiac activity — M-mode with heart rate
6. Right and left adnexa
7. Cervix (if TVS) — assess length and internal os
8. NT measurement (if appropriate gestational age)

Reporting Language

• "Single live intrauterine pregnancy. CRL 8.2 mm, consistent with 6 weeks 4 days. Fetal cardiac activity present at 118 bpm. Yolk sac visualized, normal appearance. No adnexal abnormality."
• "Gestational sac identified measuring 22 mm (MSD). No embryo identified. No cardiac activity. Yolk sac visualized measuring 4 mm. Findings are suspicious for anembryonic pregnancy — recommend follow-up imaging in 7–10 days before final determination."
• "Single live intrauterine pregnancy at 12 weeks 3 days by CRL of 61 mm. Nuchal translucency 1.8 mm (normal for gestational age). No gross structural abnormalities identified on limited first trimester survey."

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SonoBuddy's Calculators include gestational age from CRL and NT percentile reference. OB normal values are in the Measurements section.