I was just researching this. There have been several studies conducted on this topic before covid-19 (so presumably unbiased). Of each that I came across, a pregnant woman wearing an n95 mask had no effect on fetal heart rate. I did not find any study showing negative fetal effects associated with mask usage. I don't think there is cause for concern but it isn't a well studied area. I'm sure a lot more info will be available due to covid-19.
Hanged