How does baby keep from strangling on umbilical cord in utero? (not when he/she's born)?

Strangling occurs when flow of oxygen is cut off. This can't happen in utero because the baby isn't yet breathing. Something would have to cut off flow through the umbilical cord itself, which could theoretically happen, but realistically it's very unlikely, given that the cord is normally protected by a thick covering of Wharton's jelly, and not only that, but it's pressurized from within. Both things make it very difficult, if not impossible, for the cord to become compressed while the baby is still floating in the womb.