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)
| Structure | For 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 Sac | 8 mm | 20 mm |
| Fetal Pole | 16 mm | 25 mm |
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.
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.