Although ceviche is technically fish cooked with acid (e.g. lime juice), pregnant women should probably avoid eating ceviche on the slight possibility that the fish is not properly cooked, similar to...
Not just safe, it's a jolly good idea. Herring are cold water fish, so they are high in fish oils, one of which is omega-3. Also plenty of bio-available calcium and protein, and low in mercury.
And...