OB: First Trimester Dating

1. 1

   Begin transabdominal with a full bladder (curvilinear probe). Survey uterus in sagittal and transverse planes.

2. 2

   Identify gestational sac (GS) within the uterine cavity. Look for the double decidual sign to confirm IUP vs. pseudosac.

3. 3

   Identify yolk sac (confirms IUP) and embryo. Activate M-mode over the embryo to confirm cardiac activity.

4. 4

   Measure CRL: place calipers crown to rump in a neutral (not curled) position. This gives gestational age.

5. 5

   Evaluate both adnexa for masses or free fluid — adnexal ring + free fluid raises ectopic concern.

6. 6

   If transabdominal is inconclusive, proceed to transvaginal (empty bladder) for higher resolution.

7. 7

   Document GS mean diameter (3-plane average), CRL, cardiac rate, and adnexal findings.

• Longitudinal uterus (GS)
• Embryo with CRL measurement
• Adnexa bilateral
• Cervix

• Gestational sac
• Yolk sac
• Embryo (CRL)
• Fetal cardiac motion
• Adnexal masses

• Pseudogestational sac (ectopic) vs. true GS — look for double decidual sign
• Empty sac — may be too early (<5.5 weeks)

Transabdominal first, then transvaginal if unclear. CRL: straight line crown to rump, neutral position.