Because the bacteria on Mars would plausibly exist on it's own. On a different planet.
A newborn by literal definition can exist on its own. It has been born.
A newborn can breath, metabolize foods, and does not depend on being connected to another life giving organism.
The more appropriate work you're looking for is "care". You need to care for a newborn for it to survive.
You can provide care specifically for a newborn. You cannot specifically provide care for a fetus, you are providing care for the mother.
I know all of this is falling on deaf ears though.
Because the bacteria on Mars would plausibly exist on it's own. On a different planet.