There's a lot of talk about whether COVID vaccines are safe for pregnant women and their fetuses. Current guidelines, while mentioning that there's little data, suggest that they are safe. However, I'm curious about what basis there is to suspect that they might not be safe. Is there a historical precedent of a vaccine which, say, harmed the fetus? If so, what was the mechanism of action? Is there a concern about inflammation, fever, etc? The articles I find in the popular media never address what the underlying concern is.