169
First Trimester
Ultrasound
Chapter 16
Katherine Bohnert, DO
❍ What techniques may be applied to perform a first trimester ultrasound?
- Transabdominal.
- Transvaginal.
- Transperineal.
❍ What are the advantages of using transvaginal ultrasound (TVUS) versus transabdominal ultrasound?
The probe is closer to the pelvic organs, allowing a higher frequency (5–10 MHz), which improves resolution and
a full bladder is not required. Transabdominal ultrasound uses a lower frequency (3–5 MHz).
❍ What are the disadvantages of using TVUS versus transabdominal ultrasound?
- At higher resolution, a shorter distance from the probe is seen.
- TVUS is more invasive.
❍ What is the discriminatory zone?
The hCG serum level above which a gestational sac should be visualized if an IUP is present.
- The threshold level is the lowest value for which certain observation CAN be detected by ultrasound.
- The discriminatory level is the lowest value for which a certain observation should ALWAYS be detected.
❍ What is the first ultrasound landmark of pregnancy?
Gestational sac.
❍ Using TVUS, what size is the threshold for detecting a gestational sac?
2 to 3 mm.
❍ Does the presence of an intrauterine fluid-filled area confirm an intrauterine pregnancy (IUP)?
No. It could be a decidual reaction or “pseudogestational sac.”
❍ How can one calculate the gestational age from the mean sac diameter (MSD) from 5 to 11 weeks’ gestation?
- MSD (in mm) + 30 = gestational age in days.
- Example: using a gestational sac size of 6 mm, the calculated gestational age would be 36 days.