Fetus. When a fetus has reached 24 weeks estimated gestational age it is considered to be a viable fetus and therefore afforded all medical efforts to maintain life if born. Prior to this gestational age the fetus is considered to be nonviable.
At birth. Us (federal) law only recognizes life after being born. A number of states have passed laws defining life at fetal heart origination, or fetal movement, or specific times like 2nd or 3rd trimester. A few want to define a fertilized egg as being a human life. There is a strong religious anti-abortion issue here. Note that 80% of fertilized eggs pass spontaneosly - defined as abortion by some.