At what week is the typical time twins are born ???? And at what age are babies considered Preemies ?

I am 28 wks and 6 days so almost 29 wks . I have low blood pressure and on bed rest . What is the typical week twins are born Between ???And at what week can the baby's be considered preemies ??? If you don't understand my question just say so and I will try and rephrase it . Thanks every one who answers !!