I think you are speaking of gestational age. Meaning if a baby is born premature at 7 months it has abetter chance of survival than that of an 8 month gestation fetus.
This is an Urban Legend, a myth. I can find no evidence supporting it. The longer a baby is in utero , the higher the survival rate. Period.
If you are talking about a living, born baby. then you have gotten a twisted urban legend and it is also completely unfounded.