Dating scan - Fetal radiology

What is it?

A dating scan or viability scan, is a first-trimester obstetric ultrasound examination primarily performed to accurately establish the gestational age of the pregnancy.

Its findings supersede LMP-based dating if the discrepancy exceeds established criteria.

Dating scan: Why is it important.

Gestational age estimation (GA) is an essential task during the first prenatal visit.

Precise dating is important for genetic screening, scheduling the anatomical survey and ultrasound growth assessments, advising on perinatal complications, and planning the delivery timing.

Gestational dating with use of the LMP requires

• a known date of the LMP

• that the menstrual cycles occur regularly every 28 days

• the absence of abnormalities (eg. abnormal bleeding, use of hormonal birth control) in the cycle immediately preceding conception.

If these requirements are met, the due date is 280 days from the first day of the LMP.

Even when these conditions are met, LMP dating introduces error because it assumes that ovulation invariably occurs on day 14 of a 28-day cycle.

In addition, up to 50% of women do not know their LMP accurately.

Figure showing measurement of G Sac for mean sac diameter.

Objectives of Dating Scan:

1. Gestational Age and Estimated Date of Delivery (EDD

2. Assess Fetal Viability: Confirm a live intrauterine embryo/fetus by visualizing fetal cardiac activity.

3. Number of Fetuses: determine chorionicity and amnionicity 

4. Confirm Intrauterine Location: Exclude an ectopic pregnancy.

Technique to measure CRL:

• Crown-Rump length: It is the longest measurement of the embryo/fetus, excluding the yolk sac.

  
  

• Measurement Technique:

  • Fetus should be in a true mid-sagittal plane.

  • Head must be in a neutral position.

  • Calipers are placed on the outer border of the cephalic pole (crown) and the caudal pole (rump).

  • The image should be adequately magnified, occupying at least half of the screen.