Early crown-rump length. A good predictor of gestational age.

With the increased resolution of newer transabdominal and transvaginal imaging equipment, ultrasound is being used more frequently for the diagnosis of early pregnancy complications. Extrapolation of gestational age from early crown-rump lengths (CRLs) has been difficult because previously established tables of CRL versus gestational age have contained few measurements at less than seven to eight weeks from the first day of the last menses. Accordingly, the relationship between early CRL (in millimeters) and calculated menstrual age (CMA) (in days) in 36 patients with a known date of conception was studied with transvaginal sonography and found to have a linear relationship, defined by the equation CMA = 0.99 x CRL + 40.0 (r = .95, P less than .001). The relationship can be used to date pregnancies as early as 26 days after conception.