Is it safe to eat miso soup when pregnant?
Yes, absolutely. I live in Japan, my wife is Japanese and we had a healthy, happy baby boy last October. We had miso soup for breakfast just about every morning. Miso soup is a part of daily life in Japan and eaten by everybody including pregnant women.
With that said, it may depend on the kind of miso soup you are considering. If your miso has a lot of chemicals or preservatives in it, that may change things. However, if you are talking about natural miso, with soup stock and vegetables, then miso soup is probably one of the healthiest things you could eat for your baby.