The second you start considering extenuating factors, you start piling on the 'grey' area. That's probably one of the biggest problems with laws today. There's too much grey.

It should be cut and dry. A woman who is X months pregnant and is killed would be considered a double homicide.

I also disagree with your earlier statement that a fetus is part of a womans body and she 'can do with as she pleases'. I'm not so much disputing the first part of what you said, but rather the latter. There's more to it than simply saying "it's tissue". Any more depth is probably best left for a new thread. Sufice it to say that killing a pregnant woman should count as a double homicide.