How to evaluate Fetal age

First Trimester:

In the first trimester the crown-rump length is accurate to within 3 to 5 days for gestational age (GA) determination.
The measurement is plotted on a nomogram that gives a correlation for the measurement to a GA in weeks.

Second trimester:

The bi parietal diameter and femur length are most accurate; a combination of measurements is used to estimate fetal weight.  Between 14 and 26 weeks, a combination of measurements is used to approximate GA.  The bi parietal diameter is the easiest and most reproducible measurement to obtain accurately with a variation of +/- 7 to 10 days.

Femur and humerus lengths correlate strongly with both the bi parietal diameter and GA, with a variation in femur length of 7 to 11 days.  The abdominal circumference has a wider variation, +/- 2.8 weeks, and may not be useful.

Third trimester:

• After 26 weeks, all measurement are less accurate.  In the third trimester, the variation in both bi parietal diameter and femur measurements is +/- 14 to 24 days.  If a GA estimate must be done in the third trimester, serial examinations are helpful. 
• A combination of measurements is used to determine GA as the combination renders a superior result to a single measurement.  All measurements are compared to percentile values; for example, a 40 mm bi parietal diameter could represent a 16 week fetus (90^th percentile) or a 19 week fetus (10^th percentile)