There is a NUREG/CR-5631 [PNL-7445, Rev 1] titled Contribution of Maternal Radionuclide Burdens to Prenatal Radiation Doses which may answer the question of how much Sr-90 could be transferred from the mother to the fetus for incorporation into baby teeth. I haven't had time to read my copy from 1992 (other projects came up) which is an advance copy with interim recommendation but it may be helpful in the ongoing dialogue. I do not know if this NUREG was issued in a final version or has had further revisions.
Just my own thoughts on the matter
Kjell Johansen
Wisconsin Electric Power Company
Milwaukee
kjell.johansen@wepco.com