Obstetric Ultrasound

Early Pregnancy


In General
Week Mean Sac Diameter (MSD) US Findings Beta HCG
4 4 mm Gestational Sac 1,000
5 8 mm Yolk Sac 7,000
6 16 mm Embryonic pole + Fetal heart beat 11,000
Beta HCG doubles every 48 hrs.
In Detail
Week US Findings
0-4.3 No US findings
4.3-5.0 Possible small gestational sac
Possible "double decidual sac sign" (DDSS)
Possible "intradecidual sac sign" (IDSS)
5.1-5.5 ​Gestational sac should be visible by this time
5.5-6.0 Yolk sac should be visible by this time
GS should be ~6 mm in diameter
"Double bleb sign"
>6.0 Fetal pole may be identifiable on endovaginal ultrasound (1-2 mm)
FHR should be ~100-115 bpm
GS should be ~10 mm in diameter
6.5 CRL should be ~5 mm
7-8 CRL is between 11-16 mm
Cephalad and caudal poles can be identified
8-9 CRL is between 17-23 mm
Limb buds appear
Head can be seen as separate from the body
9-10 CRL is between 23-32 mm
Fetal heart rate 170-180 bpm
Fetal movement can be seen
A round hypoechoic structure in the fetal brain represents a developing embryonic/fetal rhombencephalon
Nuchal translucency may begin to be seen

First trimester

  1. Conceptus phase: 3-5 weeks
  2. Embryonic phase: 6-10 weeks
  3. fetal phase: 10-12 weeks

    Early US predominantly concerned with:

Failed Early Pregnancy

Findings diagnostic of pregnancy failure

Findings suspicious but not diagnostic of pregnancy failure

Bleeding in the first trimester

Bleeding in second and third trimesters

Placenta, fetal vessels and Uterus


Health of the Baby



Heart rate

Umbilical Artery Systolic/Diastolic ratio

Weeks Gestation Normal S/D Ratio
20 4.0
24 3.5
28 3.0
32+ 2.5

Middle cerebral artery (MCA) Resistive Index

Abdominal Cord Insertion (ACI)



Yolk Sac

Nuchal Tranlucency and Nuchal Thickening


1st Trimester Patient with +/- Bleeding and +/- Pain

You see a gestational sac

You see no gestational sac

Gestational Trophoblastic Disease (GTD)