With regard to the fetal presentation during pregnancy, three periods have been distinguished. During the first period, which lasts until the 24th gestational week, the incidence of a longitudinal lie increases, with equal proportions of<unk>  or cephalic presentations from this lie. This period is characterized by frequent changes of presentations. The fetuses in<unk>  presentation during this period have the same probability for<unk>  and cephalic presentation at delivery. During the second period, lasting from the 25th to the 35th gestational week, the incidence of cephalic presentation increases, with a proportional decrease of<unk>  presentation. The second period is characterized by a higher than random probability that the fetal presentation during this period will also be present at the time of delivery. The increase of this probability is gradual and identical for<unk>  and cephalic presentations during this period. In the third period, from the 36th gestational week onward, the incidence of cephalic and<unk>  presentations remain stable, i.e.<unk>  presentation around 3–4% and cephalic presentation approximately 95%. In the general population, incidence of<unk>  presentation at preterm corresponds to the incidence of<unk>  presentation when birth occurs. A<unk>  presentation at delivery occurs when the fetus does not turn to a cephalic presentation. This failure to change presentation can result from endogenous and exogenous factors. Endogenous factors involve fetal inability to adequately move, whereas exogenous factors refer to insufficient intrauterine space available for fetal movements. Incidence of<unk>  presentation among diseases and medical conditions with the incidence of<unk>  presentation higher than occurs in the general population, shows that the probability of<unk>  presentation is between 4% and 50%. These data are related to: # single series of medical entities # collections of series for some particular medical entity # data obtained from repeated observations under the same conditions # series of two concomitant medical conditions Rates in various medical conditions *Fetal entities: * First twin 17–30% * Second twin 28–39% * Stillborn 26% * Prader–Willi syndrome 50%, Werdnig–Hoffman syndrome 10% * Smith–Lemli–Opitz syndrome 40% * Fetal alcohol syndrome 40% * Potter anomaly 36% * Zellweger syndrome 27% * Myotonic dystrophy 21%, 13 trisomy syndrome 12% * 18 trisomy syndrome 43% * 21 trisomy syndrome 5% * de Lange syndrome 10% * Anencephalus 6–18%, Spina bifida 20–30% * Congenital hydrocephalus 24–37% * Osteogenesis imperfecta 33.3% * Amyoplasia 33.3% * Achondrogenesis 33.3% * Amelia 50% * Craniosynostosis 8% * Sacral agenesis 30.4% * Arthrogriposis multiplex congenita 33.3 * Congenital dislocation of the hip 33.3% * Hereditary sensory neuropathy type III 25% * Centronuclear myopathy 16.7% * Multiple pituitary hormone deficiency 50% * Isolated pituitary hormone deficiency 20% * Ectopic posterior pituitary gland 33.3% * Congenital bilateral perisilvian syndrome 33.3 * Symmetric fetal growth restriction 40% * Asymmetric fetal growth restriction 40% * Nonimmune hydrops fetalis 15% * Atresia ani 18.2% * Microcephalus 15.4% * Omphalocele 12.5% * Prematurity 40% *Placental and amniotic fluid entities: * Amniotic sheet perpendicular to the placenta 50% * Cornual–fundal implantation of the placenta 30% * Placenta previa 12.5% * Oligohydramnios 17% * Polyhydramnios 15.8% *Maternal entities: * Uterus arcuatus 22.6% * Uterus unicornuatus 33.3% * Uterus bicornuatus 34.8% * Uterus didelphys 30–41% * Uterus septus 45.8% * Leiomyoma uteri 9–20% * Spinal cord injury 10% * Carriers of Duchenne muscular dystrophy 17% *Combination of two medical entities: * First twin in uterus with two bodies 14.29% * Second twin in uterus with two bodies 18.52%. Also, women with previous Caesarean deliveries have a risk of<unk>  presentation at term twice that of women with previous vaginal deliveries. The highest possible probability of<unk>  presentation of 50% indicates that<unk>  presentation is a consequence of random filling of the intrauterine space, with the same probability of<unk>  and cephalic presentation in a longitudinally elongated uterus.
Breech birth
Preterm birth
Stillbirth
Coffin birth
Low birth weight
Multiple birth
Asynclitic birth
Water birth