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Ultrasound Obstetric First Trimester

USNo contrastOB-GYN

Indications

Confirm intrauterine pregnancy
Dating / gestational age
Vaginal bleeding or pain (rule out ectopic/miscarriage)
Confirm cardiac activity
Confirm number of fetuses / chorionicity
Discrepancy between size and dates

Patient prep

Transabdominal: full bladder
Transvaginal: empty bladder, consent/chaperone per policy

Contrast

None / non-contrast

Technique

Transabdominal curvilinear and/or transvaginal endocavitary transducer
Use lowest output (ALARA) — limit Doppler and dwell time over embryo
M-mode preferred to document/confirm cardiac activity rather than spectral Doppler
Sagittal and transverse imaging of uterus and adnexa

Series / Sequences

#	Series / Sequence	Plane	Notes
1	Gestational sac	Two planes	Location (intrauterine), mean sac diameter if no embryo
2	Yolk sac	Two planes	Presence/size
3	Embryo / fetal pole - crown-rump length	Mid-sagittal	CRL is most accurate dating measurement in first trimester
4	Cardiac activity	M-mode	Document heart rate with M-mode
5	Number of gestations / amnionicity-chorionicity	As needed	Document if multiple
6	Uterus and both adnexa / ovaries	Sagittal and transverse	Adnexal masses, free fluid; corpus luteum
7	Cul-de-sac	Sagittal	Free fluid
8	Nuchal translucency (if 11-14 wk screening requested)	Mid-sagittal	Standardized NT technique/credentialing required

Key points

CRL is the most accurate parameter for dating in the first trimester
Cardiac activity expected when CRL >=7 mm; gestational sac MSD >=25 mm without embryo suggests failed pregnancy
Always evaluate adnexa and cul-de-sac to exclude ectopic, especially with positive bHCG and no IUP
Use M-mode (not spectral Doppler) to document heart rate; observe ALARA

References

• AIUM Practice Parameter for the Performance of Standard Diagnostic Obstetric Ultrasound Examinations, 2024 (ACR-ACOG-AIUM-SRU) link
• Radiopaedia: First trimester ultrasound link

Source: Researched — verify against your institution

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Reference template — verify and adapt to your scanner, vendor and institution before clinical use. Not a substitute for clinical judgment.