Fasting during pregnancy is a BAD BAD BAD BAAAAD idea. The developing fetus needs all the nutrition and proteins it can get so it can get to develop properly. Then if that wasn't enough your system would be releasing toxinsthat could adversely affect the developing fetus.
It is beyond me that anyone would even consider fasting while "knowing" they are pregnant. Concentrate on healthy balanced meals including raw foods, juicing fresh veggies with an emphasis on the needs of a pregnant mother which I am sure there is lots of info out there and last but not least remaining active as tolerable or recommended for a particular stage of pregnanccy. Three things YOU and YOUR CHILD will benefit from.