I really find it hard to believe that pregnant women would assume that any psychoactive drug - in this case, cannabis - is harmless to their developing fetus.
If legalisation means increasing use by mothers, I suspect that this will be another part of a subtle, long term harm to American society that I think legalisation may be setting in progress.
There may eventually be a push back. But unfortunately, for America, the current exemplar of a drug and alcohol free life* is just about the worst possible advertisement for temperance since Hitler.
* starts with "T"