I think neshtaneh hatevah is the only way to explain these gemaras. The Gemara talks about 7 month pregnancies as if they were a miut hamotzui, i.e. a common minority. And the children born from a 7 month pregnancy were not pre-mature. They were full term on a shorter gestation.
But today, every 7 month baby (or at least almost every 7 month baby) is premature, requiring a few weeks of hospitalization.