••• Chapter 16^ First Trimester Ultrasound^171
❍ What is the embryonic disk?
The thickened region along the outermost margin of the yolk sac that becomes visible at 1 to 2 mm in length. It
correlates with a gestational age of 5 to 6 weeks.
❍ At what human chorionic gonadotropin (hCG) level will you typically see an intrauterine gestational sac?
1000 to 2000 mIU/mL.
❍ The embryonic phase of development is between and weeks’ gestation?
6 and 10.
❍ The chorionic cavity normally obliterates between and weeks’ gestation?
12 and 16.
❍ What is the discriminatory size of the gestational sac (MSD) at which one would expect to see the embryo?
- MSD = 20 mm via TVUS.
- MSD = 25 mm via transabdominal ultrasound.
❍ How can one calculate the gestational age from the crown-rump length (CRL) from 6 to 9.5 weeks’
gestation?
CRL (in mm) + 42 = gestational age in days.
❍ What is the approximate growth rate of the embryo (CRL) and the gestational sac diameter between 6 and
10 gestational weeks?
1 mm/day.
❍ Which is a more accurate indicator of gestational age: CRL or mean gestational sac diameter?
CRL.
❍ How accurate is the first trimester CRL measurement in predicting gestational age?
Within 3–5 days.
❍ When should ultrasound-established dates take preference over menstrual dating?
When the discrepancy is >7 days in the first trimester.
❍ Is it true or false that cardiac pulsations may be seen when performing TVUS before the embryo itself is
identified?
True.
❍ Cardiac activity should normally be seen when the embryonic pole has achieved what size? This corresponds
to what gestational age and what MSD?
4 to 5 mm, 6.0 to 6.5 weeks, and 13 to 18 mm.