Disclaimer: Totally not a pregnant lady. Cis-gendered male who stutters, with a wife who does NOT stutter, and we have an almost 1 year old (Edit: Who also does not stutter because she doesn't talk yet! outside of simple "mama, dada" etc). Not an expert. Second: Congratulations! Plain and simple, there's about a zillion and a half symptoms that pregnancy causes, and increased anxiety, depression, and stress can definitely happen during it. While stress isn't the cause of stuttering itself, stuttering can definitely be exacerbated by stress. Again I am absolutely 100% not an expert, but I'd bet money that's where it's coming from. She should consult her doctor.