Fetal crown-rump length or biparietal diameter for assessing gestational age in the 13th and 14th week: a comparison.

Three-hundred fetal crown-rump length (CRL) measurements in the range 10-80 mm and 245 measurements of biparietal diameter (BPD) in the range of 17-50 mm were obtained in a longitudinal study of 105 pregnancies. The width of the 95% prediction interval for age estimates was established for 10 mm ranges of CRL and BPD. The prediction interval was most narrow (+/- 4 days) in week 11 (CRL measurements from 31 to 40 mm) and from then on increased with increasing age. In the period in which both CRL and BPD could be measured, weeks 13 and 14, the prediction interval was identical for the two (+/- 6 days), so that the most feasible ultrasound measurement could be chosen.