A fetus in a dream signifies different things, depending on the interpretation. The person may be pregnant and dreaming of her baby, there may be some sort of growth or new life happening in the real-world situation.
If you are in the process of trying to get pregnant and dreaming about being pregnant means that you might want there to be some kind of emotional connection with the person who is going to father your child. If you are already in an emotional connection with the person who will father your child, then dreaming of pregnancy can signify building upon that relationship and developing more bonds.
The mom may be worrying about an unborn baby because it has been found out that she is pregnant. It might also represent feeling like they have no control over their life because the persons dreaming about a fetus are incapable of being new parents at this time.