Pregnancy, of course is not an illness. However, what is called morning sickness which results from pregnancy, certainly can cause the woman to feel quite ill. In such circumstances, food is usually the last thing that the she wants. However, if she should feel week or faint, she should take whatever food she needs (red meat if it is medically necessary), whatever she needs and whenever she needs it (even before Holy Communion). But insofar as she can observe the fasts and abstinences of the liturgical year, she ought to do so.
Fr. Vincent Serpa, O.P.