Hi, Vertex presentation is the commonest presentation. The prospects of a normal vaginal delivery are decided at term based on factors like pelvic assessment and presence or absence of cephalo-pelvic disproportion or some other significant finding. As of now, if you are worried about the presentation, you need not worry at all. The height of the baby is measured more accurately in a sonogram by certain measurements like femur length, crown-rump length and growth rate. Sonogram may be repeated if some difference in growth rate is suspected. You can consult your doctor on this matter for advice. Good luck.