OB ultrasound

The plane, and which way is up

Sagittal to the embryo or fetus: the crown-rump length is the longest straight line from the top of the head to the bottom of the rump, in a neutral (not flexed, not extended) position, excluding the yolk sac and the limbs.

Look for, in order

1. Find the midline sagittal plane of the embryo: the whole length visible, the spine along one edge.
2. Place callipers on the outer edges of the crown and the rump; take the best of three.
3. Read the gestational age from the CRL chart; the measurement is accurate to about 5 days in the first trimester.
4. Check that the sac, the yolk sac and cardiac activity are as expected for that age.

What normal looks like

A CRL of about 10 mm at 7 weeks, 17 mm at 8 weeks, 45 to 55 mm at 12 weeks; heart rate 110 to 160 after 6.5 weeks. A first-trimester CRL date that differs from the menstrual date by more than 5 to 7 days re-dates the pregnancy, and the date is then fixed.

Trap. A flexed or extended embryo, or a line that includes the yolk sac, gives a wrong length; after 14 weeks the CRL is unreliable and head and femur measurements take over.

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