I am looking for a document reference for a priest able to give dispensations. In particularly referring to fasting for pregnant woman for health reasons.
In the particular situation of a pregnant woman, she does not need a dispensation since the law of fasting does not apply to those whose health it would injure.
From the USCCB:
Q. Are there exemptions other than for age from the requirement to fast on Ash Wednesday and Good Friday?
A. Those that are excused from fast and abstinence outside the age limits include the physically or mentally ill including individuals suffering from chronic illnesses such as diabetes. Also excluded are pregnant or nursing women. In all cases, common sense should prevail, and ill persons should not further jeopardize their health by fasting.