Obstetric 1st Trimester Ultrasound ScanCoach™
View-by-View Acquisition Guidance
View-by-view acquisition guidance for first trimester obstetric ultrasound, aligned with current AIUM guidelines. Guides transabdominal and transvaginal technique with image optimization tips and normal appearance criteria for early pregnancy evaluation.
Gestational Sac
TVS preferred <7 weeks; TA with full bladder if TVS unavailable
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Transabdominal (TA): supine with a comfortably full bladder. Transvaginal (TVS): lithotomy position with the bladder emptied — a full bladder is not required and may impair TVS image quality. TVS is preferred for early first trimester (<7 weeks) due to superior resolution.
TA: midline sagittal and transverse planes through the lower uterus. TVS: insert probe gently into the anterior vaginal fornix; angle anteriorly for the uterus. Identify the uterine cavity and locate the gestational sac within the endometrium (not in the cervix or adnexa).
Gestational sac (GS) location (intrauterine vs. ectopic); GS size (mean sac diameter = [length + width + height] / 3); shape (round/oval is normal); double decidual sac sign (two concentric echogenic rings — confirms IUP); yolk sac presence (confirms IUP when visible); number of GS (multiple gestation).
Mean Sac Diameter (MSD) thresholds: GS should be visible on TVS when β-hCG >1,500–2,000 mIU/mL (discriminatory zone). A GS >25 mm MSD without a yolk sac (empty sac) is diagnostic of failed pregnancy. A GS >25 mm MSD without an embryo is also diagnostic of failed pregnancy per SMFM/ACOG criteria.
The double decidual sac sign (DDSS) — two concentric echogenic rings around the GS — is the earliest reliable sign of an intrauterine pregnancy and helps distinguish IUP from a pseudogestational sac (seen in ectopic pregnancy). A pseudogestational sac is a single echogenic ring (decidual reaction) without a true choriodecidual interface.
An interstitial (cornual) ectopic pregnancy is located in the intramural portion of the fallopian tube and may appear to be within the uterus. Key features: GS is eccentric (not central in the endometrium), surrounded by <5 mm of myometrium, and the 'interstitial line sign' may be present. Rupture risk is high — consult immediately.