Normal + Abnormal US Findings: 1st Trimester

Key Components

Pelvic Ultrasound and serum Beta-HCG

Endovaginal preferred, transabdominal for high-pelvic adnexal masses and pelvic free fluid

Gestational Sac (GS) first visualized by endovaginal ultrasound 4.5-5 weeks as 2-3 mm rounded intrauterine fluid collection

Mean sac diameter growth varies but estimated at 1.13 mm per day

Intradecidual sign - eccentrically located GS within decidua with collapsed uterine cavity highly suggestive of IUP

Intradecidual sign is BEFORE double sac sign

Absence of these does not exclude IUP

Yolk sac (YS) confirms IUP - can be visualized at 5.5 weeks as round 3-5 mm structure eccentrically within GS

Embryo at 6 weeks as 1-2 mm structure at periphery of YS

Crown-Rump Length most accurate for gestational age until 12 weeks

Embryo should be visualized when MSD at least 25 mm

Embryo in amniotic cavity (membrane thinner than YS) while YS in amniotic cavity

Cardiac pulsation at 6th week GA - absence can also be normal if embryo <= 4 mm; must be present at embryo size 7 mm or larger

7 mm CRL is necessary for specificity and positive predictive value of 100%

MSD cutoff of 25 mm without an embryo diagnostic of pregnancy failure

Ddx - early IUP, occult ectopic pregnancy and completed spontaneous abortion - need serial b-HCG

In patients who are hemodynamically stable with PUL, less harmful to wait and monitor b-HCG with repeat US than presumptively treat ectopic pregnancy