EDITOR, - Tine Brink Henriksen and Allen Wilcox1 comment on our finding that the birthweight curve in our study was steeper at term than that observed in other studies.2 We ascribed this to our use of dates based on routine ultrasound scans instead of menstrual dates to derive our weight for gestational age standard.2 They suggest that a constitutionally large fetu's gestational age is overestimated by the dating scan and that therefore bigger babies, with presumably larger birth weights at term, would selectively be plotted against later gestations, thus creating bias.

While it is true that the study they refer to showed …