Estimating the age of the human foetus from crown-rump measurements.

When data from various sources were compared, it was found that estimates of foetal age based upon crown-rump measurements varied considerably. These variations are probably related to the positions in which the foetuses were measured. Tables relating foetal age to foetal length should therefore specify the precise posture in which the foetus was placed for measurement. A standard readily reproducible position is proposed for the foetus when recording its length and it is suggested that crown-rump length should be related to "conceptual" age instead of the more conventional "menstrual" age.