4. 1st Trimester Obstetric/OB Ultrasound Protocol

Pregnancy Dating in the 1st Trimester

Mean Sac Diameter (MSD)

Prior to the appearance of a fetal pole, the mean sac diameter can be used to estimate the gestational age.

  • Step 1 – Optimize depth to see gestational sac
  • Step 2 – Obtain a sagittal view of the gestational sac
  • Step 3 – Measure the height and length of the sac using the mean sac diameter calculation package.
  • Step 4 – Rotate the probe 90º to obtain a transverse view of the gestational sac.
  • Step 5 – Measure the width of the gestational sac.

Generally, the formula for calculating gestational age from mean sac diameter is:

MSD (in mm) + 30 = Gestational age (in days)

StructureFor transvaginal ultrasound, the structure should be seen by the time the MSD is:For transabdominal ultrasound, the structure should be seen by the time the MSD is:
Yolk Sac8 mm20 mm
Fetal Pole16 mm25 mm
MSD Mean Sac Diameter Measurement OB Obstetric Obstetrical Ultrasound
How to measure the mean sac diameter in 3 planes.

Crown-Rump Length (CRL)

Once a fetal pole is present, the crown-rump length should be used to estimate the gestational age as it is the most accurate method of dating the pregnancy. The crown-rump length is defined as the measurement between the top of the head and the bottom of the torso.

  • Step 1 – Optimize depth to clearly see the entire fetal pole or embryo.
  • Step 2 – Acquire a proper view of the fetus
  • Step 3 – Measure the crown-rump length using your machine’s calculation package.
Crown Rump Length CRL Measurement OB Obstetric Obstetrical Ultrasound
Proper fetal positioning and CRL measurement.

If your ultrasound does not have an OB package use this formula: CRL (in mm) + 42 = Gestational age (in days)

Tip: Make sure that the view of the fetus is in the mid-sagittal plane. Having an oblique section or an off-center longitudinal section of the fetus will lead to underestimation of the crown-rump length.

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