surrogate mother 
noun
1A person or animal which takes on all or part of the role of mother to another person or animal.As a kid, my mom had to juggle her studies and her role as a surrogate mother to her sisters and brothers, because her mother, my grandmother, was quite mad....
- Buttons the beagle quickly adopted Monty and became his surrogate mother - a role she also played for Rommel.
- She had always seen her aunt as a surrogate mother after her mother had passed away.
2A woman who bears a child on behalf of another woman, either from her own egg fertilized by the other woman’s partner, or from the implantation in her womb of a fertilized egg from the other woman.The cell is then treated in an activation medium that allows it to develop into an embryo to be implanted in a surrogate mother's womb....
- The fertilised ovum was then implanted in the womb of the surrogate mother.
- Five more embryos were implanted into a single surrogate mother.