Crown-rump length measurements in relation to gestational age with fitted centiles (Approach 3). Full title: Crown-rump length (CRL) measurements in relation to gestational age (GA) with fitted 3rd, 50th and 97th centiles (Panel A). Panel B shows the relation between GA and CRL after interchanging the axes and fitting new models to the three sets of coordinates. Panel C shows the model obtained by simply taking the average of 2 SDs. An equation for the SD relating GA to CRL was then obtained by regressing this SD (of GA) on CRL and estimating outer centiles by combining the model for SD with that for the median (Approach 3).