Does a probability of breech presentation of more than 50% exist among diseases and medical conditions?

The aim was to study the effect of twin gestations in a uterus with 2 bodies on the probability of breech presentation at delivery. The hypothesis was that the probability of breech presentation was not higher than 50%. A review was undertaken of MEDLINE (1966-2004) and of the article reference list for statistical analysis of presentation at delivery among twins in a normal uterus, singleton gestations in a uterus with 2 bodies, and case studies of twins in a uterus with 2 bodies. There are 10 studies of twin gestations in a normal uterus (Twin A 3036 cases, breech presentation 22.36%; Twin B 2758 cases, breech presentation 36.87%), 2 studies of singleton gestations in a uterus with 2 bodies (297 cases, breech presentation 42.09%), and 57 case report studies of twin gestations in a uterus with 2 bodies (Twin A 56 cases, breech presentation 14.29%; Twin B 54 cases, breech presentation 18.52%). The odds ratio and chi-square test for differences in probabilities show a significantly lower incidence of breech presentation for twins in a uterus with 2 bodies compared with twins in a normal uterus (Twin A, odds ratio = 0.58; chi(2) = 2.08, p > .05, Twin B, odds ratio = 0.39, chi(2) = 7.67, p < .05), and singleton gestations in a uterus with 2 bodies (Twin A, odds ratio = 0.23, chi(2) = 15.51, p < .05; Twin B, odds ratio = 0.31, chi(2) = 10.72, p < .05). Twin gestations in a uterus with 2 bodies decrease the probability of breech presentation.