I've seen babies that were born six months premature, that by your definition would still have been a "fetus." With assistance from those in the outside world, it did live without the nourishment of the umbilical and the safety of the womb.
Your argument is flawed.
No baby is born 6 months early. A four month old fetus cannot survive outside the womb - and once it is born, it is not as fetus. It is a fetus while inside, and attached to the woman. Once outside the womb, its a baby.
Love modern technology, so you're hagling over the months? What is your basis for life?
I am wanting him/her/it to admit that a baby will not survive if born 6 months early.