There are two types of<unk> : breech/vertex and vertex/vertex. In breech/vertex presentations, which are much more common, the first twin is in the breech position, presenting feet-first, and the second is in the cephalic (vertex) position, presenting in the normal head-first manner. In these cases, the chin of the first twin locks behind the chin of the second twin while in the uterus or birth canal, preventing vaginal delivery. In vertex/vertex presentations, where both twins are positioned for head-first delivery, the two heads become<unk>  at the pelvic brim, preventing either fetus from passing through the pelvic inlet in a vaginal delivery. Contributing factors to the interlocking of twins include uterine hypertonicity, small fetal size, and reduced levels of amniotic fluid following rupture of the membranes. It is more likely to occur in women with large pelvises, young primigravidae (young women in their first pregnancy), and pregnancies with monoamniotic twins.
Locked twins
Conjoined twins
Vanishing twin
Monochorionic twins
Monoamniotic twins
Vestigial twin
Parasitic twin
Coffin birth