Why is Mrs. Putnam so concerned about the condition of her daughter ?

Mrs. Putnam has had most of her babies die in childbirth,

Reverend Parris, I have laid seven babies un-baptized in the earth. Believe me, sir, you never saw more hearty babies born, And yet, each would wither in my arms the very night of their birth. I have spoke nothin', but my heart has clamored intimations. And now, this year, my Ruth, my only - I see her turning strange. A secret child she has become this year, and shrivels like a sucking mouth were pullin' on her life too. And so I thought to send her to your Tituba ...

Ruth was Mrs. Putnam's only living child left.