Even though Juliet did impregnate a Male Field mouse.....it did not carry to term. Also, we do not know yet if her sister was able to carry her baby to term. So, being able to impregnate someone is not necessarily the solution to the problem. The body must not reject the embryo, and it must be able to support a growing fetus to term. The x-ray she was shown was of a uterus that seemed of advanced age and probably would not be able to carry the embryo for very long. So, my guess is that Juliet is the best hope they have, since she was interested in that type of research.....but she has not been able to come up with the results that they need yet.