The only time that happened to me my mare had a twin "dummy" foal. She gave birth to a healthy foal then delivered the other fetus which to the vet's estimate had died at about 6 months gestation. My mare had gone 11 months.My vet was sure I have the date wrong when he checked her. He told me she had two more weeks to go. So, we went on our vacation only to be called home in two days. Everything went fine, no complications.
Now vets can do ultra sounds and see it you have twins or a situation like mine.