I am no expert, but I was 10 days into a waterfast and discovered I was pregnant once. My midwife freaked and made me stop fasting immediately. That pregnancy was my easiest ever, smallest baby (I tend to grow them huge), and easiest delivery. My daughter is very healthy and almost 5 years old now. I have always wondered if the fasting was the key to such a great pregnancy! I have wondered if some waterfasting during every pregnancy would be beneficial. I think I would recommend a good antioxidant and a good amount of chlorella (as a heavy metal binder) if anyone were to try it. The chlorella offers a lot of extra protein and iron and fat and carbs and vitamins and minerals too, which would slow the fasting some, but would ensure your nutrient levels aren't too low for baby. Those are my thoughts on the matter!
Even my midwife would say it was okay for me to fast for a few days when I felt ill during pregnancy.