First Trimester Ultrasound: Timing, Purpose, and What to Expect

The initial obstetric ultrasound (dating/viability scan) is typically performed at 6–8 weeks gestation (calculated from LMP).

Primary Objectives

1. Confirm intrauterine pregnancy (rule out ectopic).
2. Establish gestational age via crown-rump length (CRL) — most accurate ±3–5 days.
3. Document cardiac activity (visible ~6 weeks transvaginally; ~7 weeks transabdominally).
4. Determine plurality (singleton vs. twins; chorionicity/amnionicity if multiple).
5. Assess adnexa for ovarian cysts, fibroids, or other incidental findings.

Early Scan (<6 Weeks)

May be indicated for:

• Uncertain LMP or irregular cycles.
• History of ectopic pregnancy or recurrent loss.
• Vaginal bleeding or pain (assess viability, location).
• ART pregnancies (confirm implantation, number).

At 5–6 weeks, only a gestational sac ± yolk sac may be seen; follow-up in 1–2 weeks confirms progression.

Scan Modalities

• Transvaginal: Higher resolution, preferred <8–10 weeks.
• Transabdominal: Adequate thereafter; full bladder helps.

Limitations

Cannot reliably detect structural anomalies before 11–14 weeks (nuchal translucency scan). Genetic screening (NIPT, combined test) is separate.

Discuss timing with your provider—individual factors may shift the optimal window.