FYI...The Midrash is NOT the Bible.
Neither is the "new testmament."
But whatever one thinks of derash interpretation, the plain sense of the text is that a very short gestation period was lengthened to a much greater amount of time. The idea that it refers to more conceptions after than before the sin is metmom's own private interpretation. Even if private interpretations are all put on the same level, her opinion is no more the obvious sense of the text as the traditional one.
That, btw, is why there has always been an authentic oral interpretive tradition. Unfortunately the Catholic Church ruined that idea for Protestants.