OB ultrasound

The plane, and which way is up

Transvaginal, sagittal: the probe is in the vagina, so the top of the screen is nearest the probe (the cervix and lower uterus), the fundus is deeper on the screen, and the bladder is at the upper left. Transabdominal sagittal is the reverse: the bladder is at the top of the screen and the uterus lies behind it.

Look for, in order

1. Is there a gestational sac in the uterus, eccentric in the upper cavity, with an echogenic (bright) rim of decidua around a black fluid centre? A central collection with no rim is more likely fluid or a pseudosac.
2. Inside the sac: the yolk sac first (a small bright ring from about 5.5 weeks), then the embryo beside it, then cardiac activity.
3. Measure the mean sac diameter early, and the crown-rump length once an embryo is seen; the CRL dates the pregnancy better than any other measurement.
4. Look at both adnexa and the pouch of Douglas for a mass or free fluid, even when the intrauterine pregnancy looks normal.

What normal looks like

By transvaginal scan, a sac at 4.5 to 5 weeks, a yolk sac by 5.5, an embryo with a heartbeat by 6 weeks. Discriminatory thresholds are conservative: no yolk sac in a sac of mean diameter 25 mm or more, or no heartbeat in an embryo of CRL 7 mm or more, is a failed pregnancy; below those, repeat the scan.

Trap. A sac without a yolk sac or embryo in a woman with a positive test is a pregnancy of unknown location until proven otherwise; an ectopic can coexist with a decidual reaction that mimics a sac.

Describe it in this order

1. The plane and the approach (transabdominal or transvaginal)
2. Location of the pregnancy: in the uterus or not
3. Number: singleton or multiple, and chorionicity
4. Viability: cardiac activity
5. Dating or growth: the measurement that fits the stage
6. Fetal anatomy, placenta, cord, fluid, cervix, as the stage allows
7. Adnexa and free fluid
8. Then, and only then, the impression