The use of femur length (FL) and biparietal diameter (BPD) for estimation of gestational age (GA) was investigated using 1123 ultrasound measurements between the 14th and 41st week of gestation. Femur length was found to provide a more accurate index than BPD for the entire range of gestational age considered. If an interval of approximately +/- 2.5 weeks is considered 'satisfactory', the accuracy of FL past the 31st week is not sufficient to justify its use. Combining the FL and BPD measurements via the formula: GA = 9.565 + (0.0414) FL + (0.0024573) FL2 + (0.15209) BPD - (0.00015827) BPD2 provides an index of gestational age accurate to approximately +/- 2.5 weeks up to the 32nd week of gestation and accurate to approximately +/- 2.75 weeks up to the 34th week of gestation. This is the first time that combining FL and BPD in a statistically rigorous manner has been proposed as an index of gestational age, and we suggest that this combination be used as a routine tool in its assessment.