Why does Mrs. Putnam contact Tituba in The Crucible?

Mrs. Putnam contacted Reverend Parris's Barbadian slave, Tituba, prior to the start of the play because she believes that "Tituba knows how to speak to the dead [...]," as she says early in Act 1.  This is important to Mrs. Putnam because she has lost seven of her eight babies to death within a day of their births, and she is anxious to make some contact with them and figure out what or who it was that killed them.  Moreover, she claims to have seen her one surviving daughter "[shrivel] like a sucking mouth were pullin' on her life too" this year.  Desperate not to lose her only child and to find out why her others died, Mrs. Putnam has come to the conclusion that witchcraft must be playing a role because there is simply no other reason that makes sense to her.  Therefore, she admits to sending her daughter, Ruth, to Tituba so that they could conjure the spirits of her dead babies and find out the identity of the witch that murdered them.