Thumb sucking habit and associated dental differences in one of monozygous twins.

A case report is presented of a set of monozygotic twins aged 14 years, one of whom sucked her thumb and one who did not indulge in the habit. The thumb sucker had a wider lower arch and a narrower upper arch than the non-thumb sucker. The thumb sucker also appeared to have a more forward position of her premaxilla on cephalometric analysis together with an increased overjet. Superimposed outlines of the tongue showed that the thumb sucker was adopting a lower position of the dorsum of the tongue at the time of initial examination.